Measure up carefully, then play with the Victorian Plumbing online planner (or similar) to trial different styles and layouts. They may not have exactly what you’re looking for, but I found it really helpful for getting a good idea of what size fixtures we could sensibly fit in the space and what was the best layout.
Consider things that make your life easier, eg we’re having toothbrush charging points inside the bathroom cabinets and de-misting mirror.
Wall hung fittings obviously help with ease of cleaning, but need careful fitting and wall strength. We are having a wall hung vanity, but a floor standing toilet for this reason.
If you are having a bath, consider depth/water capacity. Do you want to be eco or prefer the occasional luxurious deep soak? Water capacity and displaced capacity figures on specs can help you work out how deep your water can be. Also bear in mind that freestanding baths are lovely and often deeper, but a pain to clean around/behind, so only have one if your really love it.
We have teenagers, so chose items that were simpler mechanically, in the hope they will last longer without being broken. So for example, we went for a walk-in shower enclosure, rather than a sliding/hinged or pivot door.
Similarly, while I like the streamlined look of concealed fittings, we have gone for an exposed fitting shower, deck mounted taps and standard toilet cistern, as we don’t want the hassle of dismantling part of the bathroom if they leak or fail.