'Too long didn't read' version? Collusion is not a crime, and prosecutors need to prosecute crimes!
There are some misunderstandings about a Special Prosecutor's role (or any prosecutor's) as well as some confusion about what is required to impeach a President. The short answer is that there isn't really an offense or level of offense required to impeach a President. While it is true that an official can only be impeached due to "treason, bribery, or other high crimes and misdemeanors," the Supreme Court case Nixon v. United States deemed that the Supreme Court did not have the authority to determine whether the Senate had properly tried the accused official. Thus, it has become modern parlance, practice, and expectation that there really is no criteria for determining an "impeachable offense" other than what Congress deems so.
It is not necessary for Mueller, or any prosecutor, to bring satisfactory charges against or to convict an official duly impeached. This has a few potential ramifications. One is that some believe Mueller, despite never indicting Trump, could potentially "dig up enough dirt" that despite not being able to indict Trump for a crime, the Congress could still make a case for impeachment. When Mueller is on the clock, being paid with taxpayer dollars, this literally isn't his job. A prosecutor is to use the resources at his disposal in the effort to prosecute a crime, not to dig up random potentially compromising or innocuous "dirt." Another ramification, from Congress's perspective, is that articles of impeachment can be drafted at any time for almost any reason. Can you imagine the outrage over knowing which members of Congress would or would not want to oust a President without even an indictment? The precariousness of a Republic where an official could be impeached at any time, and the nonexistent charges worried about later? As an aside, I would implore Congress not to use this powerful tool, intended as a last resort, as a means of partisanship.
But back to Mueller's case itself. If he intends to convict Trump or his team of anything resembling collusion with Russian government in the election, it would have to be conspiracy. Conspiracy against the United States is defined as:
"If two or more persons conspire either to commit any offense against the United States, or to defraud the United States, or any agency thereof in any manner or for any purpose, and one or more of such persons do any act to effect the object of the conspiracy, each shall be fined under this title or imprisoned not more than five years, or both.
If, however, the offense, the commission of which is the object of the conspiracy, is a misdemeanor only, the punishment for such conspiracy shall not exceed the maximum punishment provided for such misdemeanor."
Now, this is an actual statutory violation Mueller could investigate, but why do I say the investigation is making no progress even with the recent guilty plea of Michael Flynn?
Many have alluded to the possibility that Mueller is offering reduced sentencing in return for these pleas and information on higher-up illegal activity. However, if a prosecutor wants to use this method, he has to have a plea that involves a crime for which he could prosecute someone at the top of the proverbial food chain. Then, once the person who has plead guilty and cooperated (if the prosecutor deems the guilty has cooperated substantially enough) he can advocate for the minimum sentencing for the pleaded-to crime. Simply put, if Flynn et al. didn't plead guilty to conspiracy, it's very unlikely that they will somehow be able to implicate Trump et al. in conspiracy based on their information or testimony.
The only thing that Mueller appears to have anything for at this point is a charge of lying to federal agents (which is what people who could implicate Trump have plead to!) Maybe, and I say MAYBE in all caps, he could be pressing for an obstruction of justice charge related to lying to federal agents. For the time being though, it appears Mueller is just picking up scraps content to do whatever he can to enforce the law on some unrelated fringe players.