I've no idea if this is a good idea or not but you could go another route by having blast secondarys with tweaked numbers that lowered the damage but boosted the debuff effects. If the control primarys are anything like CoH they will already be doing damage so then both powersets will be damaging to an extent and both will be manipulating the mobs. The player then has the choice to enhance damage or control/debuff on each power as they see fit.The final form for Controllers is not yet determined. They may be like Controllers or Dominators. Or they may be Controllers with 2-3 ranged attacks in their primary set. One proposal I saw was to let the Controller take a Control primary and either a buff or a DPS secondary, in effect letting the player decide whether they want to be a Controller or Dominator.
It would split the difference a bit between a CoH controller and a dominator and leave the buff sets entirely to the defenders and devastators.