Don’t move the plumbing and ventilation:
Two of the most important functional features of a bathroom are the plumbing and the ventilation. Surprise plumbing incidents can be costly, so base all renovations around the existing plumbing.
Avoid blocking the main ventilation source (a vent fan or window). If your bathroom doesn’t currently have a source of ventilation, this should be at the top of your remodeling to-do list, to be proactive about preventing mold.