Would be great if you could undo your last action. Might require a confirmation before ending your turn.
I would say the last activation of a unit. So selecting the unit, moving it, and selecting an action. I'd also be happy with a restart turn type option.
I think this is super needed, just the last used unit, after you use the next unit the previous unit's actions are set in stone
There's a damage randomizer when attacking, so while I think this would be a great improvement, it could only be used for non-combat actions.
As aiscool said, with the bit of luck in battle, it could be too advantageous to redo some actions. So I don't know. The "restart turn" will be too strong imo.
The main things I would like to be able to undo is moving a unit to the wrong square or accidentally select "wait". Pretty easy to do either of those with Switch handheld. I understand the concerns with combat. Maybe only allow the undo in single player games?