It's easier to do total overhaul mods, like Liberty City '01, since you don't have to reinvent the wheel with the core mechanics.
I believe the last two indie GTA-likes that got any traction were The Precinct and Samson, but they're mostly disappointing, except maybe for The Precinct. There are the usual issues with this kind of genre, since it's a pain in the ass to nail the sandbox, driving, missions, combat, and city design for one or a couple of indie devs, and none of them try to be as bold and edgy as Rockstar in the early days, so every attempt ends up being very sterile and mostly forgettable.
https://youtube.com/watch?v=aLAgBGjYhFU