Take V's graphics and IV's physics and stick them in another Liberty City remake with GTA VI. It's the perfect marriage. They could've done this with V but R* broke down due to all the whiners that can't handle IV's physics that require more skill to master.
GTA IV gave me 5+ years of off the wall fun. GTA V gave me two weeks.
They won't make a return to Liberty City with GTAVI, no chance, they already have created HD liberty, and even might visit it with V's dlc.
Anyway, GTAIV 5 years of fun? Because it was released 5 years ago, GTAV 2 weeks of fun? Maybe, um, because it was released two weeks ago? Well ok, maybe you didn't like it, fine, but maybe you played the game a lot, got bored of it, expected more of it, but a month later you might start it again, maybe even enjoy it, more than on your first playthrough, at least for me second and a third playthrough is always more interesting and fun that the first. Just saying, actually, just speculating.