But we would probably learn something like:
a) he's not nearly as rich as he claims
b) he's made maybe $50M this year and paid $0 in taxes due to write-offs in earlier years.
That would not go down well publicly.
Romney paid an effective rate of 14% on his $500K income (I think all capital gains) and that was destructive for his campaign. It just looks pretty out of touch - 99% of Americans earn less than $500K and pay far more than 14% in taxes.
So it would have been a publicity disaster, enough to hurt him badly.
Maybe some other skeletons in there.
But I don't think anything illegal.