It's more important that the bathroom looks clean/not broken than that it looks fashionable really. As long as it's reasonably neutral ie no avocado suite!
Definitely not wood floors in a rental as if people leave puddles (inevitable) you'll get rot or discoloured patches.
I'd go for dark tiles on the floor, non slip, large format to reduce grout lines. But make sure the floor underneath is properly solid and flat so you don't get cracking. A tile upstand (like a skirting board but shorter) looks smart and protects the walls if there are puddles.
But... are the bathrooms likely to be cold? if so then definitely go for Karndean or something rather than tiles on the floor.
On the walls I'd say off white tiles rather than stark white (but not beige either) and don't tile more than you need to (eg do the shower area and behind the sink but not more). That way you save costs and also can paint the walls in a colour that is fashionable at the time. Again large ish format tiles to reduce grout. Don't use white grout, a pale grey is much more forgiving. Maybe shiny rather than matt tiles on the walls, as will reflect light more and be easier to clean.
The mermaid boards probably are more practical - but they are not quite as smart as tiles IMO. If you don't need lots of joins (eg in a shower cubicle) they can look good, I'm not sure about around a bath where there would be more joins between the panels.
Try to have plumbing reasonably accessible so it can be fixed easily - for example exposed valve shower rather than recessed valve, and a rail shower kit rather than a fixed shower head where the pipe work is all in the wall. For sinks a pedestal sink is practical as the pipe work is easily accessible, or maybe a sink on top of a cabinet so the pipe work can be inside the cabinet. Again all easier to fix than having the piping in the wall.
period/retro style wouldn't date but is much harder to clean than modern.