1: If the multiple character setup returns, story wise maybe something along the lines of The Departed, where the characters you play as could be on opposite sides e.g. Criminal/Police/Corrupt Cop? However I personally prefer the 1 protagonist, a good, interesting, emotionally rich protagonist is all you need!
2: Although unlimited free roam is the thing that makes GTA, I think so longer story missions in different locations would be interesting and enhance the story. For example, in the game Mafia 2, there was a section where the main character went to prison for a period of time, and you had do missions such as bare knuckle boxing training, mopping the floors, fighting rival gang members and It felt like the character was going on a journey.
3: Gas Stations where you can pump your gas,pay or drive off without paying and endure the police, Cars that run out of fuel, buying cars from dealerships, selling cars legit at dealerships or selling stolen cars illegally. Its Grand Theft Auto remember
4: Can perform Impromptu bank robberies,store robberies, muggings, home invasions (burglaries), arson attacks, bar brawls. However these would all have consequences.
Bank robberies - aside from police looking for you, the surrounding areas are closed, the bank is closed for a few days, perhaps you hear on the news that the bank teller you assaulted cannot come back to work because of fear.
Muggings - depending on who you mug, time of day, how intimating you look, what weapon you have, there are multiple outcomes. Some pedestrians may try to fight you and may end up killing you instead, some may pass out from shock, some may scream alerting others, some may sit and cry after the robbery.
Home Invasion- Soon after a home invasion, If you visit a coffee shop/bar/supermarket you may hear people talking about the robbery in the neighborhood, perhaps even by the people you robbed. If you chose to murder the homeowners (tut tut tut), the house would be closed off and the neighbors would be more vocal and upset about the crime.
Bar Brawls - Start a fight in a bar with a gang member, more gang members may start throwing bottles and chairs. A rival gang may join in to help you, creating a huge bar brawl straight from the old west!
5: Improved Police AI- Instead of just smashing your car for nothing, I would like to see other methods of arresting you such as tazers, handcuffs, longer car chases.
6: Legal Work - Obviously not a main feature, similar to owning a business in V or nightclub work in TBOGT.
Taxi Driver - would need a decent driving skill level, low fighting skill (occasional unruly customers)
Nightclub Bouncer/Promoter - Would need to have a good fighting and agility skill (drunk customers, throwing out drunks)
Car Towing - Mid driving skill required
Truck driver - High driving skill
Delivery Man - decent driving skill
Car reposser - High driving skill, strength, fighting skill
Manager - If you own a business, occasional help and career changing decisions needed.
Mechanic - High driving skill, strength, (would be taking cars apart putting them in right order puzzle game style).
7: Illegal Work - Getaway Driver - Needs high driving skill
Bare knuckle Boxer - Needs high fighting, agility and strength.
Security - Accompany gang members on deals, Needs decent fighting, agility and strength.
Protection Payment Collector - Collect money, Needs decent fighting, agility and strength.
Street Racer - Mid Driving skill
Shipment Disrupter - Needs high strength, mid fighting skill, mid driving skill, mid agility, ambush cargos at the docks.
8: Health- Using different gym equipment has different effects, treadmills, sit ups and boxing training gives agility and a toned physique, while barbells, dumbbells and push ups enhance strength and bigger muscles. A food bar is included and the hole system is similar to GTA;SA system, where you character can become fat, thin, strong and fit.
9: Nighlife - over 5 nightclubs, 2 gentlemans clubs and 3 bars which are all enterable.