To get proper custom bathroom tile design one needs to ask help of the professionals because every custom alteration should be done properly. Custom bathroom is also quite an expensive thing to be done. So, you need to check your budget first.
Ceramic and porcelain tiles are the most popular and the most common to be customized, yet some people decide work with marble, limestone, natural stones or even laminate or hardwood. Ceramic and porcelain tiles are quite durable but because they have a lot of pores they are prone to the mold growth. Therefore, choosing them you need to be sure such tiles do have special anti-mold and anti-bacterial coating.
Marble tiles could be better in terms of design ideas and of course they look magnificent on photos and pictures. But their cost is high and then all your mistakes will hurt your budget for sure.