What water supply do you have? Conventional gravity system, a 'combi' boiler or a pressurised system? Do you want both a bath and a shower – or just one or the other? If you're including a shower, do you want to go the traditional route with a tray and enclosure arrangement or with the increasingly popular wet room option?
Is accessibility an issue? If so, you might want to think about a wet room or a rectangular, walk-in shower that offers more space than a corner cubicle and can accommodate a seat if it's needed.
And that's just the beginning!