What about spells with a delayed casting time? Like over several turns? Your mage uses their full action to begin casting, and they place the card in the zone with one or more “delay” counters on it. And each upkeep you remove a counter, and when the last counter is removed, the spell finishes. So you could summon very powerful creatures that are so strong that they take a couple rounds to get gated in, or a spell that has arena wide effects when it resolves.
And the trick is when these spells are started, if the mage casting it receives damage, the spell is cancelled and discarded.