I'd try it out on graph paper. You sound a little flip about where the toilet would end up -- moving them can add a huge expense to a project, so make sure you can actually fit everything the way you imagine (and that it's possible to access the waste line without tearing up the whole floor). It also seems to be that this is a big room, but moving the tub doesn't gain you any walls, which is what you need for a larger vanity, e.g. What is the opening at the top that looks like a door? If you closed it to a window, could the tub nestle there?

Eyeballing this, in the absence of any logistical constraints, I feel like bath on the right wall, shower in the corner and along the back wall, toilet in the corner opposite where it is now -- then that allows space to extend the vanity along its current wall. No idea what happens to the walk-in closet. What's that little thing behind the shower right now? a linen closet?