The gates are a little pointless since you can't turn them, can't control how far apart towers are and they don't even teleport. But it's a fully functioning 3d tower defence game so I might just be jealous ;)
Great concept, grid upgrades (to place more stuff, perhaps even linking for example weapons and gadgets) and better weapons would be my primary suggestions for a sequal. Internal upgrades (with perhaps armor in this category) would improve gameplay without too much programming. Replaying levels on a higher difficulty seems equally simple to program and again would improve gameplay.
It would be a waste of both time and money for Insomniac.