there's another theory floating about re: everyone's favorite lannister that i can clue you in on in white if you are interested...
tyrion is actually a Targ. there haven't been as many clues as the jon snow targ theories, but here's the evidence:
1. the mad king had a thing for tywin's wife, and had mentioned before that he wanted to bring back the tradition of "king's right" (basically the king gets to sleep with the wife of any of his subjects..same kind of thing happened in braveheart). it's been speculated that he raped or had an affair with tywin's wife and tyrion is the result.
2. tywin says that tyrion is lannister "until he can prove otherwise" suggesting that he had his suspicions about about tyrion's parentage and gives him even more reason to hate tyrion.
3. the dragon has 3 heads. we can assume that dany is one, jon being a targ seems obvious, this leaves tyrion to be a possible secret targ and 3rd dragon head.
4. this would also explain why varys is helping tyrion. he is a targ supporter and probably knows the truth about tyrion. why else would he risk his neck to help tyrion when he would have had nothing to gain from it?
5. patricide is a huge no no. if tywin wasn't really his dad, it would help tremendously with tyrion's image.
6. some folks have also speculated that tyrion's mismatched eyes are a result of having targ blood, but i'm not sure how his lineage would have anything to do with that, unless it's a reference to targs being inbred.
the evidence isn't as strong as jon's but i think they have some merit.