Your idea is spreading.
https://mazebert.com/forum/suggestions/time-warp--id1447/
https://mazebert.com/forum/ideas/flux-capacitor-mechanic-metro-tower--id1516/
Instead of repeating a wave to increase the number of creeps to kill, maybe the size of waves could have a chance to be increased.
I think it would work as a Cthulhu-flavored tower/item that increases the size of the cultist waves.
Implementing it as a unique tower would make it a commitment. If the tower also made cultist waves 10% stronger and increased the spawn rate, then it would be an interesting decision.
Repeating a wave feels hard to balance and maybe too strong. Making a wave bigger can make it harder, but repeating the same wave that was already survived would be a clear benefit and a boring decision.
Edit: I guess this is a bit like part of this suggestion:
https://mazebert.com/forum/ideas/hexing-scepter-legendary-item--id1530/