I think the potential for such big improvements is precisely why I don't mind if it releases this year or next.
GTA V is the only game I've really played much for the last year, and even yet tonight and for the foreseeable future, is the only game I'm regularly playing. It's sort of become my staple go-to game, and probably will remain the go-to open world (non-fantasy) game for at least a couple years. So, if they want to take an extra 3 or 6 months to make it even better, I don't mind, because the more time they spend on it, the more I'll get out of in next year and the year after that.
If I think back to my favorite games, like Final Fantasy Tactics or Deus Ex, and I really ask myself, would I accept a 1 year delay, in hindsight, so the game has even more content? It'd be pretty easy to say 'yes' once you consider you may play the game for 5 or 10 years. Granted, GTA V is not that game for everyone, nor was FFT or DE. But it is for me, and I've already beat it 8 times in just 1 year. So, if they as far as I'm concerned, take as long as you want to make it better. Every extra month is just an extra month of content.
I've no real interest in even seeing a new setting for GTA VI. I like the setting, art, and cast enough of V that I wish they'd just keep expanding and adding to San Andreas, both single-player stories and as online game. That's likely not the concensus of GTA consumers, which is totally fair, but it's how I feel at least. I think the world of GTA V is that good.