You would not have seen it yet, if Mueller is doing his job correctly.
It would not be a single ace card, but a stack of evidence, that evidence might not exist and it might, the general public does not know yet.
This is why I laugh at people who claim they know this will go one way or another on the president.
We know that he has information collected from Flynn and Papadopoulos and it was good enough to warrant a deal... but we don't know who that evidence is against or how defenative it is.
Clearly there is something yet to be disclosed... and we can assume it is important enough to warrant the deals... we dont know who its against.