The first thing to consider is location. It is cheaper to redesign around an existing bathroom, but it may be that you want a bathroom that is closer to your bedroom or is larger in size. I certainly value having a bathroom-cum-dressing room next to my bedroom and dream one day of installing the bath in the bedroom itself. Perhaps you could increase the space available in your existing bathroom by locating the loo elsewhere. This is an admirable move, since the functions of defecating and the pleasures of bathing seem at opposite ends of the spectrum. The only reason a loo is to be found in the bathroom at all is ease of plumbing; it really is better sited elsewhere. So spend some time just looking at the space you have got at your disposal - there is no rule book that says you must copy the existing location or layout. What matters is that the bathroom you design is right for your needs.
Before you make a plan, write a bathroom checklist. This is to help you prioritize what you want. Ask yourself questions like:
Is your present bathroom well located?
Is your existing plumbing providing enough hot water?
Will it still cope if you add a new fixture, such as a power shower?
How does each member of the family use the bathroom (e.g. who showers and who bathes)?
Do you need special safety provisions for children or elderly people (e.g. height of fixtures, shower thermostats, hard flooring, discreet handles)?
How is the bathroom to be heated?
How is it to be lit?
Is a shaver socket outlet to be included?
How much storage space do you have?
Some people use their bathroom for more than the obvious. They keep weights or exercise machines here. They store nappies, baby baths and other bulky items. They chuck their dirty clothes in the linen basket, pluck their eyebrows, dye their hair. They read magazines, drink wine, massage their loved ones. The choice is yours, but now is the time to think about what you want a bathroom for.