GTA games in indie market are rare, perhaps don't exist.

Some basic things to consider :
- Don't use BSP for the level , only models and LOD for all

- Use a level cutted in some sort of squarre tiles you'll only display nearest tiles, and with very low LOD the others

- Use paths for random people on streets and for cars ;You'll need a solid code that manages all that random stuff and they behave between each others and with you and AI: i think it's the most difficult part

- You don't have Rockstar engineers to make such game like GTA so perhaps make things lot more simple. people following simple

- LOD : use LOD on all objects and characters (the most LOD you have the best caus you can gain polygons between LOD A and LOD B)
- Mip Mapping caus drawing precise texturs for all objects even the far objects will ask too much power !

- LOD management for AI vehicles paths : for several ennemies you must puchase each have it's own path management When you are far from them ro don't see them you don't need to calculate

- Impostors ?

- Same thing for people not directly on screen just move them without calling animations for their displacements or even call any physics

- Streaming : you load models as you walk to areas

etc ... etc ...

Well you can see that style of game asks for lot of very advanced coding and strategy of visibility and culling !

I think for someone as you that is not a core coder or engineer its' a too much big project, just my opinion !
And alone it looks impossible, even for making all assets : several NPC ,character, items and clothing, cars all that with all LOD !!

All coding ,3D stuff etc ... too much , try to find a team
interested in making that game style : i think you can find some !

Target something lot more little for beginning or another game style than GTA !

Some ideas :
- making third personn level where you shoot zombies ?
- making a racing car where you can go everywhere like Burnout
(without needing to have combat), a level with cars only, no
characters : lot more easy to manage and make indeed !


Why not making a game that would mix theses two modes :
- You roam on car trhought the town to collect missions, buy
items etc ...
- When you enter a zone mission , the game makes a transition ,
and loads an indoor or outdoor level with ennemies and you
are in 3rd person camera for the combat mode
- After finishing or beating a boss the game reload the town
level and you are in car.

Well i stop here it was just my point of view to help you a little in your decision !