GTA San Andreas has solidified its status as one of the timeless classics of gaming. It is not only considered one of the best games in the franchise, but many count it as the best game ever to grace the PS2. GTA V has returned to San Andreas' setting, but the game has deviated greatly from its legendary predecessor.
While obviously not an inherently bad thing, as GTA V is great on its own right, these changes did cut out some of the fun parts of SA as well. Naturally, the modding community has taken it upon themselves to close the gap, as evidenced by the boatload of various CJ-in-V mods out there.
In fact, with the kinds of graphics mods being made for SA making it less aged and the number of SA feature mods being made for V, it's pretty surprising that a few modders haven't joined forces for a collaborative full conversion mod for GTA V turning it into quasi-SA.

While such a mod doesn't yet exist, you can get near enough with all the SA themed mods out there. This one adds yet another feature from the fabled game to GTA V. San Andreas had a strong focus on gang culture and turf wars, so much so that a game mechanic around this was also included.
Lucasvinbr's Gang and Turf Mod brings a system similar to the turf war system in San Andreas. This mod allows you to create your own gang, name it, pick the members, equip them and fight against other gangs. The map of GTA V has been separated into control zones which you can take control of by pushing out enemy gangs.