Ah, the OT tap dancing superpowerz have returned. Truth doesn't matter; protect the fake PR image at all costs. Familiar pattern.
It's simply not true that there is anything at all preventing Trump from releasing his tax returns. The "If we pretend hard enough, our delusions will come true" only works on cult followers and Trump supporters.
Until he does (eventually) we can only speculate. Maybe there are some skeletons in the cupboard, but we don't know at this point. The media speculated a lot about Obama's birth certificate but it was released there wasn't much to discuss after that.
Until it's cleared up that he didn't snort coke before the debate, we can only speculate about his drug abuse and mental instability. Until he releases his tax returns, we can only speculate as to what he's hiding.