While there is no telling how vindictive Democrats might be in pursuing Trump over his tax returns, the fact that he has them professionally prepared insulates him to a large degree from criminal allegations as long as he did not override the advice of his legal counsel when preparing those returns or hide things from them.

So while people like to speculate, it is unlikely that any criminal allegations come out of a tax audit and if they do they will likely collapse somewhere in the legal process. In addition criminal cases can take years to wind their way through courts and by that time who knows who will be President and if the cases appear to be vindictive a Republican President could gain a great deal of support by simply ending them with a pardon.

That said, most wealthy and corporations take aggressive tax positions which often fail in tax court resulting in back taxes and interest, but as long as the tax position was reasonable even if wrong, it does not constitute fraud. Democrats will claim these aggressive tax positions are fraudulent (they are not) and the media will run with it.

On the other allegations, like business practices and sexual assault accusations, it is unlikely they will go anywhere.