Heroes 5.5 WarGame Edition - Tactical Edition.

Hmm... a radical, but interesting idea.

It's not that radical – just remember an ability like Inspiration, which is free and reusable.
But for Inspiration, you pay with a level, so even though it's free, it will be quite powerful (50% shift), while ordinary spells costing 0 mana will be much weaker than paid ones.
Specifically – 60% of the power of spells costing 1 mana.