[Foxy]

*Deeper/Crazier portal/gravity puzzles. I thought these were underdone in Prey. I thought up a load, like:

A lift shaft with a portal at the bottom that exits at the top of the shaft. You fall in from the lift shaft entrance and it's a bottomless pit. 'aaaaah'. To get out you activate a portal in the middle of the shaft that leads out.

A boss that you have to defeat by arranging portals so that a mega-destructive energy beamed is channeled through them towards it.

*Better combat.
*More level variety