I'm sure it will be possible. But I'm not sure about its success, because after all, GTA: Online will make most of the playerbase stick with it. A mod like SAMP for GTAV should offer a lot of advantages in order to attract people. What advantages? I don't really know, maybe new and custom game modes, higher amount of players per server, etc.
I will say it again, I'd LOVE it if there was something like SAMP's Partyserver in Online, with turfwars and persistent competition, not simple TDMs, Planetside 2 style.