EDIT: As of now, I am updating the OP with the recommendations people make in replies that I agree are good additions. I think this is a great opportunity to build a truly comprehensive list of features that we should demand of Rockstar be applied to GTA V. My greatest fear is that GTA V is going to be sidelined as Rockstar pours 110% of their time and attention into expanding GTA: Online. While I think we all understand GTA:O is a massive undertaking that will require an insane amount of work to build up, I don't think a single one of us advocates leaving GTA V for dead, withering away on the side of the road from neglect and its failure to realize its full potential. GTA V is a phenomenal game that deserves to rise up to greatness along side GTA:O, not stand in its shadow...
Right now, it's my opinion that there is a split between GTA: Online and GTA: V that I feel should not exist. There's a lot of fun things you can do in GTA V that you can't do in Online, and there's a lot of fun things in Online that you can't do in GTA: V. Considering a majority of features that I am about to list WERE originally promised to us as features available in GTA V and not just GTA:O, I see no reason why Rockstar shouldn't implement and port them.
1) Buying apartments
Rockstar has stated that buying apartments will not be implemented into singleplayer because it would cause conflicts with the story. Perhaps that's true for Michael and Trevor, Michael because a family man doesn't buy an apartment and live separate from his family and Trevor because he hates the idea of living in Los Santos or any place OTHER than Sandy Shores. But perhaps make the feature available at LEAST for Franklin. Franklin has no romantic involvement, no family to tend to, no preconceptions about living in a more developed area than Grove Street (obviously since he was THRILLED to receive a cliffside villa as a gift from Lester)...there is no way that purchasing an alternate penthouse apartment would conflict with Franklin's character or lifestyle.
2) Larger garages
This is Grand Theft AUTO. Emphasis on the Auto. GTA 4: TBOGT allowed us to save up to 16 vehicles. The maximum vehicle storage space in GTA V is only 6 per character, 18 total. With over 250 vehicles in the database, it's absurd that we can collect only a small fraction of them in a game whose very NOMENCLATURE suggests car theft. Opening up Online garage locations in single-player to ALL characters is the one way in which Rockstar can truly live up to that kind of a game title. The other thing is that said garages should be available to ANY character. As of right now, I have to switch to Franklin to drive my muscle cars, Michael to drive my sports and super cars, and Trevor to drive my trucks and offroad vehicles. It's a hassle and incredibly annoying, especially when I want to go offroading, but then I find myself half-naked in a dress and drunk at the top of Mount Chiliad. My plans now have to wait for me to hike down a mountain, go back to my trailer, change out of my cross-dressing attire, and then drive all the way down to South Los Santos to fetch my Sandking from Trevor's garage...
3) Unique Jobs
Yes, we have the ability to purchase properties for a source of income in GTA V, but that income is only available once per week. GTA V could, potentially in future DLC, implement a job system that is similar to GTA: Online's system, but based on the contacts and acquaintances of the SP characters. For example, Lester can give jobs to Franklin, Ron can give jobs to Trevor, and Michael can get jobs from the studio or from Dave (let's say Dave gets into trouble and needs Michael's help for whatever reason). It's do-able.
4) Car Insurance
As of right now, there is absolutely NO way to safeguard your prized SP vehicles from total destruction. While aircraft, boats, and helicopters do respawn back at their spawn points, cars do not. If your Adder (Bugatti) is destroyed in a police chase, you are out $1,000,000 with no chance of seeing your car or that money again. Mors Mutual should absolutely be added to GTA V.
5) Persistent vehicle spawning
Similar to GTA: Online, GTA V should have a system where the last vehicle a player entered of any class (and by class I mean helicopter, boat, car, etc.) will have its location saved and marked, so that a player can stray more than a half-block from said location without having to worry about it disappearing. I have seen this system work EASILY in GTA 4 using a trainer, so I can't imagine it would be difficult to do in GTA V. It would be extremely helpful for the purpose of car hunting. Let's say you're in a Corvette. You like your Corvette. In fact, you LOVE it. And you see that nice, shiny Tesla Roadster zoom past you. You want to grab that Tesla Roadster and bring it to your garage, but the problem is you'd have to forfeit your Corvette to do so. You can't expect players to only use their character's DEFAULT personal vehicles so that, in the event of such a discovery, they aren't forced to simultaneously forfeit another prized possession.
6) Expanded car customization
GTA: Online has several features in Los Santos Customs that should be implemented into GTA V, namely the ability to install ignition or remote car bombs, crew emblems, and pearlescent shades for metallic paint, which greatly adds to the ways in which you can create a truly unique vehicle. While the options are, as of right now, still far more deep and various as compared to the lackluster Pay-N-Spray system from GTA IV, there's no good reason to hold back all the customization of GTA: Online from GTA V.
7) The expanded purchasable car database; Making the Stinger GT purchasable
I don't know about anyone else here, but I've got a love affair with two kinds of cars: American muscle and classic sports cars. With the addition of additional purchasable vehicles to the LegendaryMotorsport and SanAndreasSupersport web pages (in game, of course), players have a far easier time acquiring their favorite cars. While, up until this point, I have come to know it that finding your favorite ride in GTA is earned, it takes sometimes DAYS of hunting to find, which makes acquiring it feel that much more rewarding. However, I do not feel that there should be any vehicle in GTA: Online that ISN'T available in single-player. While I would understand if Rockstar would be unwilling to fully reconcile the catalogs for the dealership websites between Online and SP, I think at least making the Stinger GT (not just the standard version) available through LegendaryMotorsport.com is a reasonable request. I know there are plenty of people that would love to get their hands on this car, but don't want to have to spend their hard-earned ONLINE cash to do so.
8) Calling Lester to lose a wanted level, locate vehicles, etc.
Let me get this straight. For a random schmuck who pretty much JUST arrived in LS, Lester is willing to go through the hassle of scrambling a false APB to divert police attention away from said schmuck, but for people like Trevor, Franklin, and Michael that Lester has grown very close to and considers his friends....he can't do the same? The act of NOT having those features in SP conflicts directly with the development of friendship amongst Lester and the main characters IMO. It's the little things like this that can go a long way to adding much-needed depth and a sense of completion to the singleplayer experience.
Other Suggestions (From Replies on this Thread)
Revoemag & Mind4k3r: Mechanic services to deliver your vehicles to you at any location in the event you become stranded.
This is an EXCELLENT recommendation. You should not have to resort to cheat codes OR a long hike back to civilization. Too many mission end locations leave you way too off the beaten path and lightyears away from the nearest road.
TGPomy: Calling Merryweather for backup and air support outside of missions.
This would DEFINITELY have to be endgame content due to the recurring conflict between the main characters and Merryweather Security, but beyond that, I can see this being extremely helpful. We had Dwayne Forge's goons as a backup option in GTA 4, there's definitely that void that needs to be filled. When you're about to tackle a highly dangerous operation on your own, you need some friends to watch your back. While calling Michael and Trevor to "hang out" falls in sort of along those lines, it would be AWESOME to wield the power of airstrikes and targeted attacks on individual targets in single-player. Imagine the look on the Ballas' face when they realize they're about to have an AGM crammed down their throats!
N.W.D.V.: Add the locations of convenience stores to the map.
This one is perhaps the biggest common sense move Rockstar could make. You have to rob at least one store to get 100% completion, so by technicality, it is an essential part of the SP experience. It SHOULD be marked by waypoints, especially since it's one of the easiest ways to heal up since there are a good number of them spread out throughout the map. Sure beats the hike all the way back to your safehouse.
willyum32: Accessible inventory
Perhaps one of the most functional and helpful features I've seen in GTA:O! It would certainly make a GREAT addition to GTA V. It would be incredibly convenient to be able to carry a few cans of Sprunk with you for a quick health boost when you're in desperate need of it. It certainly beats the hike back to your safehouse for a healthpack.
1) Simeon vehicle repo missions at the beginning of the game for Franklin
2) The return of TBOGT-style Drug Wars through adding Gerald as a contact.
3) Gang raids/gang attacks.
4) The addition of Online weapon tints/skins to SP Ammu-Nation stores.
5) Custom Races
All excellent ideas, IMO!
Doing repo work for Simeon at the game's start can give the player an opportunity to make a few extra bucks to play with later on for when your missions drive you to invest in the stock market. It's an idea that can end up paying dividends.
TBOGT Drug Wars certainly had its flaws. They were repetitive, random, and, by later missions, INSANELY difficult. Rockstar would need to look at the mistakes they made with Drug Wars' iteration in TBOGT, but if they did and they worked on it, I think Drug Wars would make a GREAT way to earn a side income in SP, as well as level up your skills. Plus, I will ALWAYS advocate the expansion of end-game content. The more the merrier.
Gang attacks is nearly COMMON-SENSE. Since firing your weapon or acting aggressive in a gang area in SP already triggers a gang ambush, it should at least be structured and perhaps offer a mid-sized cash reward at the end to give the player SOME sort of reward for surviving such often highly lethal encounters.
Adding the Online weapon tints to SP is also a no-brainer. Very easy to do. No reason why it shouldn't happen.
I couldn't agree with this more. Custom races are SORELY needed in SP. With only 5 street races to grind out, it can seem like a bad investment to spend 150K-175K on a dedicating racing build when you only have a handful of races to put it to the test in. Custom races would be a great source of alternate income and, in a more elementary context, a great diversion and side activity.
Survival could possibly happen. We have Rampages for Trevor, but nothing similar for Michael or Franklin. Survival would be a great way to balance that out, possibly basing the background of each survival on a random encounter/conflict with an antagonist from one of the game's main enemy factions such as the Ballas or the IAA. The possibilities are nearly endless. Great suggestion.
In the end, it's the same game, the same engine, the same dynamics. They should BOTH provide the same (or at least nearly the same) experience. Thoughts?