I'm not sure. I wasn't a fan of the DL feature back when it was announced and I'm still not a fan today. It simply reminds me too much of MTA - abysmal download speeds, inconsistent HUDs, models that don't fit the GTA:SA lore and so on. Sure, I can see people wanting this but I'm not confident saying that this won't ruin the GTA:SA gameplay experience for some people, even if we're talking about the minority here. I personally think that it's too much freedom on the developers part. You either greatly enhance the experience (not gonna deny that) or fuck it up royally.

I think allowing custom skins, pickups and map icons would be a great idea. I would advise against custom vehicles and custom objects, though.
If you want to somehow modify a vehicle, attaching objects to it should be more than enough. As for objects, I like the idea of having community-driven "object collection updates". Set up an official platform for everyone to showcase their models and let people rate them. Then include the best-rated objects in the next release.

I'm happy to discuss this further, feel free to share your opinions.