There's one thing I would absolutely love to see back in GTA: the Smaw aka homing missile launcher. that thing is beast.
Also I wish there was more ways to clean out your cash pile, e.g, rent, girlfriend, court cases, being robbed. Make it necessary for players to keep the money coming in.
AND also, I wish there was a place like a safe house, where the cops can't go like the compound on godfather 2, only with anti air craft missiles and guards who shoot cops on sight. Maybe you have to earn the right to use the place, and the cops can use their brain and find vantage points and use sniper rifles too. They try to destroy your anti air missiles so their helicopters can lower cops into the compound to hunt you down. I wish the compound was designed like a fortress that's FUN to defend. With high walls and walkways like the liberty city prison. This would be crazy fun on multiplayer.
You have to buy the compound to access it, pay the guards, choose how many guards you want, have different types of guards (with different weapons and levels of armour, imagine a juggernaut suit). You can also upgrade the compounds defences, like having to pay for your Sam turrets, snipers tower/perch, guard dogs and CCTV etc.
This would be great for having cop wars and remember in San Andreas where you could get in debt with the casinos and they'd send death squads after you, I loved that, imagine they send better death squads though, with armoured vehicles and buzzards plus they used some sort of ambush tactics in essence tried as hard as the cops to kill you, even harder maybe. OMG I need to stop giving up free ideas.
Sorry for the wall of texts.
You have a safe in the safe house where you stash money and you're able to keep money on you, you have to have money on you to spend it, so you can choose weather to pay the casinos or not pay them without actually being broke, if the death squad kills you outside your compound, they take what you have on you. If the death squad kills you and your guards at the compound, they raid your safe and take what you owe, plus more (you know how people are). If you don't have the money, they can wreck your place, cause you to have to pay to fix/unlock things in your compound, like your safe, armoury, vehicles, Sam turrets, guards, sniper tower etc. Same with the cops, if they kill or capture you on the compound, they disable your illegal assets like sam turrets, armoury and impound vehicles.
Also it'd be cool if you could buy AND sell assets such as homes, businesses and vehicles etc, make the prices change too dependant on condition if its vehicles and just randomly go up and down over time if its a home, and if its a business the price depends on how well it's doing obviously, and how it's doing depends on how much input the player has in its operations, like you can rob a warehouse that stocks a product your business needs to boost profits. Make it possible to negotiate a price when buying and selling, this might be going a bit deep but I was thinking of the player being able to sabotage a business belonging to someone else to lower the price.
I think the law system should be up to date too, like if I commit a crime in front of a CCTV camera there's a good chance the cops will surprise me with a visit in the near future, and you won't know what it's for until they tell you. (Or you can use the computer in police vehicles to see what exactly you're wanted for)
Imagine the cops come to your safe house to take you in, and instead of your guards just opening fire and declaring war, how about you have the option to go hear out the arresting officer, or look at their arrest warrent through your security gate and decide weather to kill 'em all, flee or let them take you in. That would be beast. I was thinking, this compound should have a guarded dock, helipad and underground escape route that'll take you out into the sticks.
If you get taken in or killed by the cops they should make you pay a fine dependant on your crimes.
Also, you should be able to bribe a cop to remove any APB's out on you, this cop often requires a favour instead of cash payments.
Also, can't forget about cage fights. YEEEEEAAAAH!!!!