So a bit of history here. It's been said that a sitting President cannot be indicted and prosecuted. That is a legal theory that has never actually been tested. Maybe, maybe not. But what was been tested is this.
A sitting Vice President can be. That is exactly what happened to Spiro Agnew and is why he resigned (he later plead guilty).
That meant Nixon needed to appoint a new VP. And that new VP required Senate confirmation. That's where Gerry Ford came from - a centrist moderate that people could live with. Had Ford not pardoned Nixon, he might have been re-elected.
So carry that forward. If Pence is indicted and has to resign, his replacement will be like Ford. And once that happens the field is clear to remove Trump.
Mueller knows all that. So the request for White House documents related to Flynn is possibly really aimed at Pence. For the reasons outlined above.
"According to The New York Times, Nixon "sought advice from senior Congressional leaders about a replacement. The advice was unanimous. 'We gave Nixon no choice but Ford,' House Speaker Carl Albert recalled later"