Just a small collection of ideas I have:

[li]Achievements
Way more Achievements.
Maybe some "Fun"-Achievements, that just happen randomly or other things.[/li]
[li]Time Trial
Some sort of Timetrialmode[/li]
[li]Introduction of a Pointsystem
Wouldn't it be funny to get/lose Points for movements and stuff? :)[/li]
[li]Puzzle Mode
All in all the Game is a Puzzle itself, but why not makin' a mode where you have to rebuild a structure? Maybe some abstract ones and some "cool" ones. Ever thought about a Truck made of Goo?[/li]
[li]Morale-Goo
Something I thought when I built high towers.
Why not having Goo that will get scared if the tower starts to move to hard?
If it gets too scared it loses grip and BAM ^^[/li]
[li]Live Towers
When you're in Sandboxmode to build a high tower with the extra goo, it would be cool, if you would see other players which are live playing, too, at the moment[/li]