So the most serious thing as far as Trump is concerned, as far as I see it, is the Moscow Project cooperation. Most people are saying it’s the campaign finance violations which Cohen told investigators Trump is complicit in, but I don’t believe for a second that Trump would face serious problems over that, and here’s why:
They would have to prove Trump knew it was a violation in order for it to rise beyond the level of a simple fine. Obama violated campaign finance laws to the tune of $1.9 million, but it was simply a paperwork error. He didn’t intend to violate the law, so he was handed a fine, something to the tune of $350,000 iirc. In campaign finance law, it needs to be proven that one knowingly violated the law in order for it to be a criminal matter, as opposed to a civil one. Proving that Trump was aware that payments to Stormy Daniels were campaign finance violations is going to be difficult, I believe. The labyrinthine payment methods could very easily be described as discretion, hiding an embarrassing matter. Especially considering Trump had a long history of doing this, unrelated to any elections.
The Moscow Project, on the other hand, could be serious. On it’s face, it’s unseemly and questionable, but not illegal. However, if Mueller can prove that there was some sort of quid pro quo between Trump’s campaign and Russian officials involved in the Moscow Project, some sort of agreement to help Trump in his election in order to help the Moscow Project come to fruition, that could absolutely be seen as a conspiracy with Russia - collusion. In the absence of this quid pro quo, it goes nowhere, but the possibility hangs out there like a dark cloud.