If I had my own place, I wouldn't install towel radiators at all. They are seldom good enough to heat most bathrooms, the ones we have in our rental look fancy but every towel ever slides off them (see pic), so I have to put velcro strips on them, and last winter I discovered each one (and we have three) was using more than our entire day's electricity consumption. They were using about 8-10kwh each, so we were spending about a tenner a day on bloody towel rails. If you are the sort of person who remembers to turn them off and then on an hour before using a bathroom, fine. But I don't think you will get enough heat out of them after an hour to warm most bathrooms.
I think they also run off water and heat up when the upstairs heating is on, which is useful WHEN you actually have the heating on, but otherwise they are just an attractive money pit, and you still have a cold bathroom when your thermostat thinks your other spaces are warm enough. As our house is so well insulated, the upstairs heating is off more than on. So we have cold bathrooms.
I dislike the wall mounted ones as there always seems to be something about the nooks in the rungs design that causes them to rust.
If you live in a high limescale area, be wary of dark colours. We have completely abandoned two showers because the limescale that builds up on the dark grey panels is intolerable.
Rainfall shower heads are great, but they also use an absolute ton of water. If we both use it briefly, it completely empties our tank of hot water. Oh, how I dream of immersion boilers.
You can get extractors that turn themselves off fifteen minutes after you turn the light off, we have them.
If you live in the countryside, make sure you get ones with in-built fly mesh.
I agree with a PP, rounded hardwear is easier to clean than squared.
I would resist getting a toilet or sink that actually touches the floor if I could. Cleaning the bottom of those is a pet hate. The toilet fixtures and their coverings always seem to rust, too. But if you chose a design that doesn't touch the floor, you have to bear access in mind in case a plumber needs to problem solve.
Don't forget to design space to put your shampoos, etc around the bath.
Staying the obvious here, but if you install a glass shower door, make sure there's a rubber stop on the bath panel to stop door handles slamming into it.
Before installing toilet roll holders, put a roll on one, get someone to hold it in place on the wall, and all sit on the loo and practice using it. Two of ours are just ever too slightly far away which leads to being difficult to tear the paper off without ending up with it unravelling, or contorting to make sure that doesn't happen. So now they don't get used.
If you have tall people in your family, get them to sit on the toilet before it is installed. My OH has to sit at weird angles on one where it's just too close to the wall AND a sink.
If your house is prone to movement, don't get floor tiles with grout. The grout will crack. Lino is more sensible.
Don't put a rolltop bath close to a wall, it's a menace cleaning behind them and access when they go wrong is hard.
Think about where your bath sits versus where your windows are. You don't want to have to stand in the bath to open a window.
Fancy dado rails with lots of grooves and decorative fancy skirting boards are all very nice to look at, as long as you don't have to clean them.
(most of my rental house seems to have been designed by someone who has never actually lived in a house before)