do you have a nice budget?
First thing I'd do is raise the ceiling over the shower and remove the soffit. hopefully that HVAC vent could be relocated, or moved up w/the ceiling.
Take everything out and gut it to the studs.
make any repairs, move electrical or plumbing if needed.
If you do a wall mount faucet, it will give you a tad more room on that vanity. I recently did my neighbors bathroom. Same size as yours (maybe 1/2' wider. theirs is 5'x8')
got them a 36" vanity w/a right offset (the sink is slightly off to the right to allow more countertop space on the left side)
Notice the wall mount faucet and how it also gives you extra room.
It's also a lot easier to keep clean.
if you do a longer niche, you can have all your shower products here.
do you want to keep your tower? how much storage do you actually need?
you could do a cabinet over the toilet, or these shelves.
I've seen this recessed into the wall between the studs for some shallow type of storage.
get a vanity with drawers, and do a recessed, larger medicine cabinet for your main mirror to maximize every bit of storage.
this mirrored medicine cab opens on both sides. and the cool inset shelf along the entire span also gives you extra storage for smaller jars and bottles.
modern type of box shelving above toilet painted the same as the wall.
pick a vanity with maximum storage options
if you can recess this 'tower' into the wall and borrow a bit of room, this would be great^
if you can borrow space from another room in order to get another foot, you could do a built in cabinet^
Q