Adui
While it’s outside the scope of choices that you provided, clearly the next concept expanded to boundary should be Conquest itself. For the Integration of Dimensions into himself is its own Conquest, sublimating planes of existence into his very will and power. But barring that, I think integrating all Nine Dimensions would naturally give rise to the Boundary of Dimensions.
it may be unpopular, but I think a boundary involving the Animus summons would be ideal for one fact. The main villain at this point in time has a devouring aspect to him, and we know the cutest blue slime who was the best at it at one point in time. Conquest would be optimal for obvious reasons, but I also think a boundary relating to the seven deadly sins ability would be a refresher.