In terms of mtg, anything like "Monstrosity" or "Renown".
Flavorwise, becoming monstrous is subjective in terms of size and strength, though renown is okay for landing a hit on the enemy, but-
Mechanically, having to remember whether or not something is renown or monstrous is not difficult, but it's ugly because the marker they decided to use to represent the status change are simply +1/+1 counters; considering those are used for almost anything, mixing them up can be a bit convoluted.
Also, transform cards to go and stay go. I hate having to obtain two copies of a card, use ugly proxy cards, or having to physically remove the (potentially valuable) card from the sleeve; none of those options are appealing.