No concealed cisterns. They are such a pain if something needs to be fixed, and therefore also harder to adapt for water saving strategies.
(Have never installed one but lived in a place which already had one.)
Position over-bath showers properly and take account of adequate room to move around under them. No one ever told me to check this. Once ended up with one that was slightly off centre, and even if you were skinny, you had to keep one arm pretty much by your side when showering.
Don't position traditional taps very far back from the edge of the sink so that if trying to fill a cup from them, you often bash it against the porcelain.
Do tile all way up the walls around a bath with shower. The more tiles the better all over really, stops worrying about splashes messing up paintwork.
I am really not convinced the standard plastic shower screens give enough coverage, plenty of splashes go past them.
Get enough shelves around the sink area to suit your needs. If there's room. I don't know why shelving on either sides of a bathroom sink is not more of a thing. Much more useful than a ledge in front of a mirror where stuff can easily get knocked over.
And get shelves that are easy to clean (minimum of crevices to get mouldy) and don't go rusty (like that cheap chrome a lot of wire bathroom shelving is made of)
Floors: I don't like lino because water gets through gaps and soaks floorboards, but OTOH, stone or tile floors are a nuisance to take up if a repair needs to be done.
LOVE underfloor heating. Makes such a difference to comfort in the winter.
For an upstairs bathroom, a nice big sink in which it's easy to get a glass or cup under the tap, and a plastic bowl in the basin.
The most water efficient taps and showers and loos would, personally, be a priority. There are much better ranges available than there used to be with a good variety of designs
Multi-bulb light fittings are very in but I find them wasteful, would go for something with a single bulb unless the room was very big, then two spaced out. And if possible, something that didn't highlight every single dead midge in the fitting.
Heated towel rails more useful and attractive than a flat plain radiator