Donald Trump could very well be hiding something in his taxes from the public eye. It is no secret that a large portion of American millionaires have been practicing tax evasion and other types of financial loopholes and fraud. Judging from Trump's track record on financial integrity it is very well to assume that he could be hiding some aspects of his financial empire. Trump's personality can imply that if he did not have anything to hide he would boast about it and everyone would be in the know.
Trump should have already released his tax information. The only reasonable explanation for him to withhold his financial information is that there is something he doesn't want the public to know. As a wealthy and successful businessman who knows how to work bureaucratic systems to his advantage, Trump is likely hiding unsavoury information in his tax records.
This is yet another reason Trump is not to be (for lack of a better pun) trusTED.
If Trump kept to his original ideals of 'build the wall', 'cut taxes', 'bring jobs back' (not sure how), 'rebuild the military' (needed), 'reduce regulations' (this is needed in places like California) - then he would be fine. Unfortunately for him (maybe fortunate for us), we get to see the wolf in sheep's clothing, even though we have effectively thrown the baby out with the bathwater and have been burning the bridges behind us. I'm sure that Trump has something that would be used against him in the general election - such as investments in the Clinton foundation, Saudi Arabia, China, etc. which would kill his narrative.
On a more juicy part, people will comb through his records, looking for 'smut' and companies that he deals with and either harass them or extort them. Politics are ugly. It's possible that Trump has scandalous entries in there - if he WANTS to be the president, he'll have to release them.
I strongly believe there is no way that Donald Trump is concealing anything at all having to do with his tax records. Like him or hate him the Donald is intelligent and an astute businessman. I do not see how he could possibly be foolish enough to hide anything that may be discovered at a later date.