Streets of Rage 4 or any Streets of Rage official remake. Given the way the Street Fighters/King of Fighters/Fatal Fury characters progressed I'd love to see what Sega would have done with upgraded versions of the SOR heroes and villains.

Traps & Treasures 2 - planned for 1996 but never made, which is a shame as the original is one of the most inventive and fun platform/puzzlers I've played.

A new Crash Bandicoot game/games. This is a great franchise which needs a reboot.