Well since the PS4 is roughly 10X more powerful than last-gen then the game should be able to pack way more content and enhancements, here's a list of what I think should be added, list your own ideas!
1)Texture details should bee doubled if not tippled!
2)Resolution should be increase!
3)Darker nights, better stealth mechanics!
4)View distance increases(Sniping people shouldn't look so blotchy far away)
5)More Buildings to go inside.
6)More missions and side missions.
7)New content like guns and gun attachments(lasers!?), cars and car parts, and clothes.
8)More random encounters, like maybe catch a serial killer or meet an old GTA character.
9)Better car spawns, see more types of vehicles, I still haven't found the Sabre muscle car yet.
10)Since the updated game doesn't come out till fall I hope to see a massive game DLC, one that adds more story missions or even new characters!