Whaat??
It's the whole point of the wonder. The bonus with free colosseums is just there to provide a bonus if you don't or can't have vassals in a game.
UUs can be very useful, even if they are not your original UU. If not else, you can join them to a GG, their bonuses remain even when you upgrade.
And of course it's a very cool flavor IMO, makes your wonder really unique.
So, if you don't want to add the UU feature, than don't add the wonder at all.
It would just be another boring new wonder without any uniqueness, unfortunatly many can be seen in other mods.
If you can get the UU features to work properly, I would definietly add it (along with Topkapi Palace)
If they work, then it won't be an add just for the sake of adding wonders at all.
Again, I think the free Colosseums and the strengthens vassals bonuses are the extra bonuses for the UU and UB wonders, the main bonus is UU/UB features, not the other way around.
EDIT: Sry if this post is a mess. I already had a couple shots of absinthe tonight
EDIT2: I have the same concerns as EdgeCrusher about wonder obsolation. What will happen then to the free colosseums? I think it should never obsolete.
I can imagine a civ watching his neighbors closely, and trying to train something very similar to the enemies unique units. The 20-30% chance represents that trying, and of course this can happen through the whole game...