Time travel/alteration should be off limits. Unless you put them under a ridiculous amount of restrictions, as I did with my own time controller in Bitp. (To the point that it hardly made sense)

The limitations were something like that she could not affect any living organisms while having time stopped, and that she could only speed up time, not aging or the like. And more of them, I forget them all. But they were numerous, arbitrary, and confusing. It is far easier to say that unless we can somehow make them A) make sense and B) be balanced, time powers are off limits.

As for time stopping, it might be okay if s/he cannot stop people (only non-living things.) Or if s/he cannot harm a being while stopping time, and that is requires concentration. But again, time stopping/travel/alteration is really skirting the lines of what is balanced.