GTA IV was intended to be more realistic, which is why things like jetpacks and insane weapons such as the flamethrower and minigun weren't included. The next GTA may not try to be as realistic, so they may return.
Rockstar said that Planes weren't included because, since Liberty City is only one city, there's nowhere to fly to. However, a lot of players were sad to see this feature go, so it will probably return in the next GTA.
You can vote in the poll to choose your most-missed feature that wasn't in GTA IV.