If Trump obstructed justice, what next?

The latest Washington parlour game is trying to determine whether or not special counsel Robert Mueller is currently investigating Donald Trump for obstruction of justice in the Russia election meddling inquiry.
Mr Trump's lawyers have denied it. The president himself seemed to confirm it. Mr Mueller isn't talking. All this speculation misses the bigger question, however. What happens if Mr Mueller not only is looking into possible criminal misdeeds by the president, but he ends up finding them?
Such a prospect opens a legal Pandora's box - but it's one Mr Mueller should probably be pondering. Here are some possible what-next scenarios if that smoking gun guns turns up.