If we're looking for a wide-open topic, how about the alphabet? Customizers pick their letter and are free to represent that letter any way they choose - from simply having the letter on a Minimate chest, to replacing the head of the Minimate with the actual letter, to painting the Minimate as an object that starts with that letter (your letter is Q, so you paint a Minimate in a quilt pattern). Lots of options for a lot of different skill levels.
And classic Gene Colan Dracula, the Son of Satan, Satana, Lilith - Dracula's Daughter, Manphibian, Bloodstone, even Franken-Castle. There's a lot of classic Marvel horror characters I'd love to see in Minimate form.
Longtime Undertaker manager Paul Bearer has passed away. He was one of my favorite managers when I was young - the perfect compliment to the deadly serious and lifeless Undertaker character.
I'd go with a standard chest for Absorbing Man - he usually has an average build when he's not using his absorbing power. And purple pants.
And I love your design for his face.