Other than airplanes, which has pretty much already been confirmed... I just want the army back. GTA IV lost a lot of points simply because the FBI was 5-6 stars worthy, they only used to be 5 stars worthy. The army has always been in GTA and it shit me the find out they were taken out. I love the mayhem the army brings. If they do bring the army back, there has to be tanks and these tanks can't blow up a car by ramming it. It needs to crush it.
It also showed in the trailer that weapon modification may be in, with silenced ak47s. I'd love to be able to attach silencers to weapons, I have always wanted to do that. GTA IV lacked a silenced pistol, which would've been pretty fun in many cases. Silencers are just what I want.
No annoying needy friends, I really wasn't a big fan of in-game friends simply because they're always calling at the wrong times. It would be nice to disable them, rather than putting the phone on sleep mode.
Street racing needs to be back, with vehicle modifications.
Considering there's a country side, I'm expecting there to be some off-road vehicles. I want some 4-wheelers, dune buggies and 4X4s (all 4 wheels to spin, instead of the only rear 2).
Bike physics, I can't say that I was a massive fan with the bike physics in GTA IV, TLAD and TBoGT... They seem too sluggish and floaty.
Gravity, I think it needs a bit of an increase in downforce.
Vehicle suspension, way too much body roll. Needs to be reduced.
Thats all I'm hoping for in GTA V, everything else is pretty much fine for me.