We're looking at this very same thing right now, our toilet is separate to bathroom 1, which has separate shower and bath and we are thinking about combining the rooms plus shower and bath so we can enlarge out en suite, which is next to it.

You can get some very modern, funky shower/bath combos these days! I thought I wanted to keep it separate but for the benefit of adding a toilet to our en suite, we will probably put one in.