I would like to see a tick box added to Edit Pitcher Settings that would let you designate a bullpen pitcher as a spot starter.

If a rotation pitcher's turn came around and he was not at his designated fatigue level rather than skipping to the next rotation starter, the designated spot starter would take the turn.

Mostly this will help with minor league management where you might not notice right away that a pitcher's fatigue level is low.
Often skipping one starter's turn and making everyone else pitch a day sooner quickly wrecks the entire rotation.

This is a really good idea.

Could be made one step more simple by automatically starting the most rested LRA / LRB if the next SP is below 100 (or a level we can set).

I know I'd check all my LRs as Spot Starters in my MinL. Wouldn't care which one simmy picked. And your right that logic like this would prevent a lot of P fatigue issues.