I really like BCalchet's suggestion, it means that the towers complement your strategy rather than being one all of their own. The more I read this thread the more I think of Legacy of Kain and the pillars of Nozgoth with their guardians, they were (see if I can do it off the top of my head...):
Pillar of...
- States
- Dimension
- Nature
- Conflict
- Death
- Mind
- Time
- Balance
And one other, can't remember what (too lazy to google). Maybe a solution is to make the towers cheaper but one for each mana type and thereby make the effects specific (that way you could give them benefits that are useful in their own right thereby incentivising the towers AND help make the TOM victory potentially attractive). I agree that having them as world wonders makes things a lot more interesting but you'd DEFINITELY need a good info screen to help you with this. I still can't believe Civ generally has no facility to let you see which city has which wonder beyond the top 5, given the mana attached to some of them this becomes even more iportant!
God I love this mod...