Don't cut the ceiling joists without consulting a structural engineer. They in fact help support the roof because they form a structural triangle with the rafters that directly support the roof.
If you want to avoid the structural issues you can leave the joists in place as exposed beams in your bathroom. You will need to clean them up and stain or paint them or cover them with additional layers of wood to give them the beam look that you want.
Otherwise talk to a structural engineer first. He will have some good advice for your project.