Right now, Controllers face three problems:
1). Their primary ability -- Control -- is pretty much irrelevant during the end-game, as well as during most TFs. Groups of lower-level enemies will die in seconds, and AVs are prohibitively difficult to hard-control (though they can still be debuffed).
2). If you do want hard Control, Dominators will always be better at it, since most of them will have permanent Domination.
3). Subjectively speaking, "Overpower" just doesn't feel very impactful (to me personally), due to its randomness.
Proposed solution: change the way Overpower works. Instead of granting a chance for a +1 Magnitude effect, grant a global Magnitude bonus to your Control effects, based on how many of your Control effects are layered onto the target. This way, casting a single Hold onto an enemy may not Hold them; however, casting Hold and Disorient and Fear and Sleep might push them over the edge into being Held, due to the bonus from 4 different Control effects. A nice side-effect of this solution is that it makes Sleep useful in large groups (as a Control-booster, at least).