I think the urban environment provided by San Fierro will bring forth great change for gang roleplay, it sort of enforces a modern mindset if you think about it. You can't get the same effect from other cities like Los Santos -- their infrastructure and the way they are modeled, their textures, everything about them just screams "old and outdated", hence why a lot of factions there tend to opt out for more old school concepts, even ones that are already obsolete IRL. I don't think we're gonna see that in San Fierro, it's likely going to be more realistic and modern.
And let's be real here, you don't need a war-torn, poverty-stricken favela to roleplay a gang in, that's just not how things work in real life.
Although most gang hoods do look a little rough with some graffiti here and there, you wouldn't be able to distinguish a lot of them from normal neighborhoods. I mean, if you don't believe me, just go watch videos or something, everything looks normal and civilized apart from some broken fences and graffiti. You gotta remember that a gang in real life on its own is not a community, it's a mere sub-group -- it may be influential, but it's no way a majority. Most people that would be living in a neighborhood are civilians and would rather stay away from crime, they have families to take care of, not everyone is brave/stupid/unlucky enough to risk it all.
No sane neighborhood or church council would let the appearance of their home deteriorate into some post-apocalyptic ghetto with crack dens everywhere, super old infrastructure, etc. San Fierro is good as it is, really, we don't need Central-American ghettos from the 80s so that we can have proper gang roleplay. We don't even need special mapping -- some graffiti here and there would be nice along with trash (e.g. chairs and couches scattered throughout the alleys, etc), yeah, but that's all, just keep it vanilla. And for reference, here's a map with all the gang-controlled areas in GTA:SA singleplayer:

Do you see Ocean Flats or Santa Flora marked up on there, what about all the other neighborhoods that some of y'all said are the only ones suitable for gang roleplay? Nope, none -- Rockstar highlighted everything else instead. What this means is that practically all areas in San Fierro are suitable for gang roleplay, especially when you take into account the fact that we'll likely have apartment complexes spread around the city. Also, do remember that San Fierro has literally the best and most realistic street grid in San Andreas, this will allow for some very intense corner and gang beef roleplay, like who claims what block, etc.
I don't know about you guys, but I'm hyped as fuck to roleplay a gang member in SF. The atmosphere, the buildings, everything is just so much better than LS and LV.