Start with plumbing reality
The location of existing drain and supply lines can heavily influence where the bathroom belongs. Moving plumbing or adding pumps and new drains changes the scope.
Ventilation and moisture matter
A below-grade bathroom needs a plan for exhaust, water-resistant materials in wet areas, and good moisture management around the rest of the basement.
Coordinate finishes after the rough work
Tile, shower or tub configuration, vanity size, lighting, storage, and fixtures should be selected with the underlying plumbing and electrical layout in mind.