Mueller will try to make a case against President Trump for something. He may even procure an actual indictment from a DC grand jury against a sitting president, so that the Democrats/MSM can make hay against Trump if he challenges the constitutionality of indicting a sitting president.
I’m guessing the delay in the sentencing of Mike Flynn is to postpone the sentencing until after Trump is interviewed by Mueller.
That way, they can bring the transcript of the interview with Trump to Flynn to read in order to encourage Flynn to contradict something Trump said, on the offer of reduced sentencing.