That would be amazing!

You'd probably have a slot limit based on rarity level. So you couldn't have every ability on every card. Based on existing cards, slots would probably 2-4, but maybe expand it to 3-5. It would probably also make sense that abilities could only be combined upward. So a common ability could be added to an epic monster, but not vice versa.

This would create huge extra demand for summoners as well.

And yes, NFT trading would definitely be required.