I like monstrosity a lot. Magic gains a lot from having these sort of "Once per game" effects, and there's a lot that wasn't used in Theros with this mechanic because it was specifically about vertical growth.
For a while I've had this card which has been surprisingly tame for blue savannah lions:
Like sure, Monstrosity is post-hoc kicker: why not let it have a spell like effect?
But we can go deeper:
Now I'm not entirely sure I like this card, but the idea is there: The first time it deals damage, it gets a counter. You could also have him gain flying at that point, or lifelink, or something but that's a bit too much to cram on at this CMC. It's already a very easy to achieve 3/3 for 2 in black that can block.
This really sent me thinking though. What if we did this?
The 5 was always a bit much to activate unless it was the only thing you were doing, and when people did activate it in the lategame they usually had enough mana to play whatever they were returning, so this is less along the lines of "Actually letting people cast what they return" and more "Expanding their options if the need is pressing".
Might want to be reduced for noncreature spells only, but I could go either way on that one. It does come down and activate for 5, which is a bit weird.
Then I started thinking, what if we started applying this template to other existing creatures?
From:
The cards aren't that different, but the low end is a bit nicer, you can just cast it for
lategame and get your creature, and it still has that bloodthirsty flavor in there.
It's a little akward with the odd dodge of the double reminder text, since I'd need to combine the reminder text for bloodthirst and monstrostiy in the same set of brackets, so I just spelled it out.
Again, numbers aside (Given the hour I made these they're all likely too strong) I think there's probably some cool space here. It's more text, but with pretty intuitive gameplay behind it that could be worth exploring.
Edit: Wait shit I forgot the most important one:
