If in the far future an Entity that exists in the Corp Cube (Top level cube for Workflow) wants to breakout into its own cube, what is the best way to handle this?

If they want the cubes to be extensible, then I believe they'd need to do it now. Else it's just a completely different cube that shares dimensions. There would need to be a DM job or a connector rule written to import the data from the stand alone cube to the Corp Cube as they won't be connected via the Entity structure.

Extensibility within the Entity dimension is different than other dimensions - it's handled on the cube. So if EntityA is in the MainEntity dimension for Phase I and the requirement is to have a separate cube for EntityA for Phase II, then it's too late if you want to reuse the EntityA name. You could, of course, set up EntityB in a new dimension and attach it to a separate, extended cube and move data from EntityA to EntityB. But if both cubes use MainEntity as the entity dimension, there's no link between them. Take a look at the GolfStream cube and look at the Cube References tab - these are the cubes that are linked to GolfStream.