I would go with Hovak physic because I refuse to buy a PPU unless i get another ATi card with the drivers working.
maybe make the Sp longer and with better replay value.
The main problem i had with PREY was the MP part. While it was a blast for 2 days it got boring to fast. MP is what keeps a game alive if there is an almost non existing mod com. So throw in more maps and modes.
Not to eager on DX10 but why not...just make sure it's still DX9 compatible.
Don't make some dumb open ended game where people get the feeling they played only half a game...and definitly don't go episodic on us.
Well, better graphics should be a no brainer for the future. maybe more weapons that arn't standart gears with different names.
last but least put my name in the credits...or dedicate it to me