Rei

That's probably the best option, actually
Interested in any yordles from the other games, like Legends of Runeterra for example? Or sticking to just the main game ones you're comfortable with?

Kennen and Heimer for sure, perhaps Amumu if he's still regarded as a yordle? Could try something creative/different with him.

Would suggest Mega Gnar but you might need a larger chart just for him~

How about Gnar? I bet he'd look amazing in your style~ Perhaps with only some parts of him having gone Mega sized~