There must be something pretty damning in those taxes...
I think that depends on what you call damning. We already know he doesn't pay any income taxes. That was shown by the one page that was leaked, and he confirmed it during the debates. He's been using paper losses from his mismanaged real estate developments to offset income since the 1990s. He said this makes him "smart".
We also know he doesn't make any charitable donations. He admitted as much during the debate, and his foundation's books show what he hasn't made any personal contributions since 2005 or something.
And we already know that he's not really a multi-billionaire. He's admitted as much on television. His business income mostly comes from licensing his name to other companies, for a fee, and he has used today's low bond rates to convert his annual licensing payments into an assumed equivalent fortune. Bond rates are extremely low, so a few million per year in licensing fees is "worth" billions of dollars, in the eyes of his accountant. He doesn't actually own anything of value other than his name that is worth that much.
But those aren't even the real reason people want to see to see his taxes. We already know that US banks stopped lending him money after his third bankruptcy, and that his corporate empire has been financed by foreign (mostly Russian) banks since then. But his taxes returns would give some indication of exactly how much
money he's been given, and thus how much leverage foreign (maybe state-owned) banks have over his personal fortunes. That's the real reason I think his tax returns will come out eventually, as part of the investigation into Russian influence over the US political system.