Based on Sheldon’s t-shirts, it seems that he is partial to DC and not Marvel, but this Big Bang Theory Xmen mashup by Axel Medellin is a humorous take on the characters.
You’ll have to click and enlarge the image to read the lines, which make up half the fun. The other half is the artwork, which I think is cool in many respects, although the likenesses are sometimes not that clear (especially that of Penny).
Btw, Axel says a lot of his fellow comic folk feel offended by The Big Bang Theory, and if you’re one of them, get ready to be offended; as for the rest of us who love TBBT, enjoy!
If you’re into Marvel: