I really want a Prince of Persia-esque mod which allows local time effects at a cost of some sort (I haven't got a clue what, but it probably shouldn't be automatable). And yes I am aware that there is a mod that does some of this (can't remember the name atm), but not the important bits which would be localized time slow down, time acceleration and time stop, all of which could be easily done by changing tick rate. And this has been done in Dartcraft, but was in no way balanced (create time torch, then never bother with it again).
An additional feature would make this all the more interesting, PvP interaction. Basically, if one player activates an fast time, and the other activates slow, they cancel out (potentially harmfully), or if they both activate the same the effect doubles. This would allow for some very interesting combat, as double speed would require lightening reflexes while slow fights would probably be boring, so a high tier item might allow immunity to time effects. The time effects would also make interesting traps, such as by procing wither or fire, then accelerating it so that it does damage really fast. The mild downside to that would be acceleration of regen, which might need to be specifically avoided for BM purposes.