What risk? It's a DLC! How is there risk involved?
I used to like the idea but now I'm against it. GTA just isn't right for zombies or aliens. Maybe Red Dead Redemption turned out good when Rockstar took the risk of Undead Nightmare, but Rockstar wouldn't take that risk on something as big as the GTA franchise. I can see them actually making a separate game about killing zombies or aliens before doing a zombie/alien DLC for GTA V.
Unless you're talking about the risk of not making profits off of it.
The risk of people claiming it ruined the game/series and also not making a profit.
I don't see how it would ruin the game. You fight aliens in a hallucination when on drugs and get abducted by them while on drugs.
If you played Undead Nightmare the whole way through, it's basically pretty obvious that it was an actual NIGHTMARE, i.e., something that only happened in a dream Marston has. So why would a DLC adventure where you fought aliens in Los Santos/Blaine County in a dream (or while high on drugs) ruin the game?
Really, though, I'm hoping more for a DLC where you play as Trevor and have to stop the invasion of Killer Klowns (from Outer Space?)
On the other hand: GTA has never been about sci-fi crap (the alien sh*t in V was drug induced), so therefor it wouldn't feel right.
Red Dead wasn't anything about sci-fi, either. It's true that they might have been more willing to experiment with that franchise since it was only two games in instead of V but again, I don't think introducing a "what-if" sci-fi premise for GTA V would be impossible for us to accept (especially since within the game itself you can find UFOs and even actual ghosts).