Bathrooms pack plumbing, waterproofing, ventilation, and tile into a small space, which is why a small room can carry a big renovation cost. Doing the hidden work right is what prevents leaks and mould down the road.
Waterproofing matters most
Behind the tile, proper waterproofing membranes and correctly sloped shower pans keep water out of the structure. This is the part you can't see and can't cut corners on — water damage from a bad shower is expensive and hidden.
Ventilation
A properly sized exhaust fan vented to the outside (not into the attic) removes moisture that would otherwise feed mould and peel paint. It's a code requirement and a small cost with big payoff.
Layout and budget
Keeping fixtures roughly where they are saves on plumbing. Moving the toilet, tub, or vanity adds cost. Budget for the trades you can't DIY — plumbing and electrical — and keep a contingency for what's found when walls open up.