I think, but I'm not entirely convinced, that limiting spells to one spell per turn (and free cantrips) might be the best way to go, for balances sake. It seems likely to me that extra attacks will be implemented as extra actions (just like the extra attack from Haste is implemented right now), and the idea of an EK throwing out 5 (action, action, Surge, action, action, Haste action) spells in one turn -- 5/6thsof their spell slots at level 7 -- is a bit much, even if it's a niche case.
Another reasonable way to limit it, in my mind, that would let casters use more magic on their turn without enabling EK (or fighter/caster multiclasses) carpet bombing would be to limit it to one spell per action type. So a Hasted wizard could potentially Misty Step (bonus action) away from melee, throw a Fireball (action), but then not be able to throw a second Fireball with his Haste action but have to lob a cantrip at the enemy instead.
