It'll be fun listening to the right-wingers whimper about this after what happened with the Starr investigation.
You refer to a "pivot" and perhaps that's fair, in the basketball sense, where a player holding the ball can legally pivot around a set point. If the issue being investigated is Russia's interference in our election, then there are various connected issues that can be reached all around that, without "lifting the pivot foot." There's the question of collusion and the question of cover-up. Obstruction of justice related to that issue is one such near-by matter that can be fairly reached by pivoting.
What I don't want to see is what that Republican hack, Ken Starr did. In basketball terms, he basically held the ball and ran all over the court looking for dirt. Eventually, an investigation that was nominally based on a small-time 1970s land deal in Arkansas somehow got to be entirely about the question of whether or not the president had obstructed justice with regard to attempts to pry into his 1990s sex life. That kind of "traveling" is an abuse of special prosecutor authority, and should not occur. Although it would be hilarious to see similar tactics used against the Republicans after they tried that stunt with Clinton, it would be inappropriate. What isn't inappropriate, though, is investigating alleged obstruction of justice that's directly related to the original matter of the investigation -- Russian interference with the election to get Trump elected.
So, if Mueller starts to look into, say, Trump's mafia ties in the 1980s, or his alleged rape of his wife, or his violation of the Cuba embargo in the late 90s, or some sexual tryst with an intern that hasn't even occurred yet, etc., that would be an abuse of power and I'd hope we could all align against that. But if he's looking into the firing of Comey (after Trump admitted he was thinking about the Russia thing when he decided to do it), or lies under oath by Trump associates about Russia contacts, or shady deals with Russian oligarchs, or off-shore banking deals that funneled funds into the Trump empire from Putin connections, or payments made to guys like Flynn, Page, Manofort, etc., that's all fair game. As long as there's a reasonable nexus with Russia, it's a legitimate pivot.