Any time you move plumbing or drain lines you're talking big $$$$. I would put the tub where the shower is, perhaps a free standing tub with one of those circular shower curtains? The rest is just finding a style you like. Search Houzz products for sink, vanity, etc.

My approach to addressing remodeling projects starts with how each client uses the space, storage solutions and ergonomics of daily use. I focus more on this information first and once this is addressed then I start designing the space and specifying materials.

In your case, I would agree that moving the existing plumbing in this size space will most likely have no benefit to the overall layout and will increase costs. With that said without knowing the size of the shower, I would try to work with a 60" built in tub to create a tub and shower combination. Also, you should keep the tub lower so you will have no issues stepping in or out of the tub when showering. I would use a frameless clear glass tub sliding door enclosure this will eliminate doors opening an closing in a tight space. Keep clear glass and no framing on the doors to allow for a open feeling which will enlarge the space.

I would keep the height of the vanity to 36" and add a roll out drawer on the bottom of the vanity try to add drawers if possible and full height doors with no dummy front on the vanity this will help with access to storage on the bottom of the vanity and make it much easier then bending down and tying to dig items out of the base of the vanity. Also the 36" height will be much more comfortable for standing and bending in front of the sink.

Try to incorporate GFI electrical outlets into the inside design of the vanity to keep hair dyer savers etc. plugged in for daily use when needed. Add holsters to cabinet doors or drawers to hold these items and keep them off the countertop areas, this will insurers keeping the space neater.
Toilet should also be a comfort new height. All these recommendations work with the ergonomics of using this space.

From this photo, you most likely have and open wall in front of the toilet. If that is the case, you can add a closet by opening the wall to 60" high and making the depth to the width of the studs about 4"deep. Add adjustable shelfs and a door and you have a huge storage solution for less than the cost of a finished cabinet.

You can see a lot more thought goes into these spaces than just making a pretty bathroom. Hope this helps!