I think there should be a tower that can move freely. I mean every 3 seconds, let's say, it move in an empty spot. By moving it gains an additional bonus (damage, crit, crit chance or whatever). The teleportation speed cannot be shortened. So basically I have no idea on what this could be, but I kinda like this idea. I think it should be a gold one tho.
A cerimonial tower. You give it something (example: 4 items and 1000g, or 4 towers and 4000g) and it gives a random tower something in exchange: bonus, potion, handicap (example: loses 10% crit chance and damage or keep it busy for ten seconds). However I wouldn't like this tower for the sole fact it needs a lot of implementation. Even though it's alike the Horadric cube this choose between more categories and not always it gives you what you would like or expect.
Owl - it gains X% experience with a scientist in range (+ Z% for each scientist in range) and has Y% of missing an attack with a scarecrow in rage (+ Z% for each scarecrow in range).
I think it make a little bit of sense since the owl is known to be Athena's symbol and since Athena is the goddess of wisdom and the scarecrow part well, scarecrow is supposed to scare bird doesn't it? :)
Hope you get some inspiration at least ^^