Congress Can Legally Make Donald Trump Himself Testify Before Them And They Should Do That
Congress legally has the power to compel testimony and documents (including recordings) from Donald Trump himself.
The easiest way is to compel documents from entities he controls. This was used against Michael Flynn. He refused to hand over documents personally, but complied after his companies were forced to turn over documents. In Trump's case think about his businesses tax returns and other records. Are there any Trump University or Trump Foundation information that could be useful? This could be a gold mine for investigators.
Secondly, Trump himself would be treated the same as any other citizen with the exception of things that relate to his presidency. Things like executive privilege would work as a shield in certain areas. However, it wouldn't apply to anything that happened before his presidency. So decades of business deals and campaign stuff are fair game.
So Trump can legally be called and would have to testify, but what if he just ignores it. Well, congress would have to use it's inherent contempt power to fine him and/or imprison him until he testifies or hands documents over.
He can fight document handovers on fifth amendment grounds but his entities must hand them over and he can be held in contempt for getting his entities to not hand them over. Such documents to handover include tapes, tax returns, business records, and communications with an Alfa Bank server.
He doesn't have to answer questions but congress can require him to appear and can ask him questions. At that point shit gets serious because he will most likely go apeshit if you goad him. Therefore he will fight back and if he's combative the Democrats can get a lot out of him. Remember he secretly wants a spectacle too since he's a camera whore and would love nothing else than to be the center of attention in congressional hearings. At that point it will be easy to get him to incriminate or damage himself.
Congress, what are you waiting for?