There are only 4 Spells in the Player's Handbook that are reactions (No material outside of the Player's Handbook has been confirmed):
Shield, Hellish Rebuke, Feather Fall , and Counterspell .
As readying an action will likely not be implemented the only other use is Opportunity Attack.
I think it's a little dramatic to call 4 spells and attack of opportunities a core part of the systems balance.
Shield is automatic for mages when hit. They don't know how much damage they are going to take and it covers them until the start of their next turn.
Hellish Rebuke is also automatic. When you're hit you use it.
Feather Fall is also automatic. If you're falling you use it.
Counterspell is a choice.
As a mage if I had all 4 spells prepared I would prioritize Shield, then Feather Fall, then Counterspell, then Hellish Rebuke, then Attack of Opportunity.
For all other classes Attack of Opportunity is automatic. I've never saved one in case someone else moves.
Is it worth devoting a serious amount of development time to overhauling the engine to devise a system that gives mages more agency with respect to exactly 4 spells?
I'm not convinced.
I'll be playing the hell out of Early Access and if it turns out that I find it annoying or that I burn through spells too quickly I will definitely provide that feedback.
As of now however I don't know if a system overhaul is worth it for four spells.
