Maybe some cards can get upgrades that aren’t evolutions. Maybe some cards get added abilities, like champions.
Maybe others can get a ‘morph’ mechanic. Here is a thought on that:
If you have both freeze and rage in hand, you can play morph for 5 elixir that freezes and rages selected area at same time OR there is a slot (like evo slot) where if you have rage there, it lets you had a card like freeze to play with morphed card instead of just rage.
Graveyard can morph with goblins for undead goblins to rise
Ice wizard can morph with Magic Archer for a chill arrow
Etc…