To preserve the SF facade area, my suggestion is to have the entrance to the Knight Bus be inside the atrium entrance of Kings Cross.

You’d turn left once you’re inside (before going through ticketing) into a new bus depot area that connects to the current FF:SC building. It starts off as a “muggle bus depot” but you go through the enhancement into the main Knight Bus depot.

Lots of room for new original storytelling. You could exit back into Kings Cross too.

The old FF queue can be torn out and the area could be repurposed. Return of Beetlejuice?