Since you're rebuilding the whole room, moisture and ventilation cause the most trouble later. Put in a fan sized for the room and vent it straight outside, not into the attic, and run supply and drain lines so the fittings stay reachable. Behind tile at the shower and tub, use a continuous waterproof membrane and check that the shower floor slopes toward the drain before the tile goes down. Check door swings and vanity clearances on paper too, since those are hard to change after framing. Is the shower on an exterior wall, and where is the tub in your layout?