For a "Controller" role to exist, content throughout the game - from ten/fifteen up to fifty - would have to be adjusted in such a way to be dangerous enough to mandate its existance. Anywhere groups come up, for it to be an actual role as opposed to what it currently is - a handful of abilities on a handful of classes - it would have to have presence and be part of 'the formula'.

It'd be a massive undertaking to restructure, and to be completely honest I'm not entirely sure I'd call it all that valuable.