I'm thinking mainly about strong spellcasters caught by surprise by a large group, as is possible in quests in .981, but I also think something along these lines could do something to help the 'dullness' problem with the current spell set.

The details could work out in several ways as far as costs go, but the general idea is to make 'summon' a short-term (tactical) class of spells and rename the current summoning spells as binding spells (or summon & bind, if the UI has room for that much text).

This should work separately from however the Enchantment slots vs. cast-once-only stuff that's current in .981, enabling players with a summoned and bound Fire Giant to summon an additional Fire Giant for one-time duty in tactical combat.

High-level, high-essence casters should be able to summon cannon fodder to the tactical field as well, choosing to deploy several weaker units instead of a single stronger one.