What do you think of this?:

Powerful mages can use master spells such as elemental storms, mass healing, summon atrocity, etc, but to do so requires preparation. When they begin the spell a meter or countdown appears above their head to warn the player that baddery is imminent. If this is too intrusive then they simply enter a casting stance or special animation that the player must watch for. The player can interrupt the casting of the spell with various interception methods: knockdowns, freezes, stuns, critical interference range shots, paralysis, frenzy, drain magic. Calm has the opposite effect on them, they can cast the spell under calm and the casting time is shortened. In the reverse case, if your companion mage is calmed then their spell will apply first to counter or overide before the enemy mage completes their casting.

And during this, of course, the battle area has erupted with harassment from other non-magic foes and your warrior companions are busy entertaining them.

If they add some form of "charging" magic attacks to scale based on how long you spend "charging" and same with physical attacks there should also be all kinds of ways to interrupt both. That would give the powerful attacks a risk to reward ratio instead of being all reward. I would find it much more entertaining.

I love that idea. I think there should be only very few capable of this however, and maybe a good mage character could reach that level as well. In TES lore, you read about mages of that power, but you never see it.