We have his and hers vanities that each have towers with adjustable shelves, outlets, and a valance. I prefer to have just the pretty countertop accessories out so I designed my side with all concealed storage. I'm right-handed so the things I use each day are on the right side cabinet, toiletries etc on the top and hairdryer & curling brushes on the bottom. The left side has storage for some misc bathroom accessories and my contact lens supplies on the top. My curling iron and brushes fit on the bottom. My husband's vanity and towers are very similar but he preferered to have open shelves on the lower half of his towers so that the things he uses each day are in view. I love threads like this because there are always some really creative ideas that I would have never thought of!

It looks like it was added at some point. And it extends into a space behind it. What do you want to store? I’d think another built-in with drawers below and a cabinet with shelves above would provide good storage for bathroom items.

Get the largest vanity that will fit. Racks over the toilet. Here is my very small guest bath in my Florida home. That bottom drawer hold 8 hand towels. We have a linen closet down the hall from this bathroom. When we bought this home, the previous owner had two towel barson the door plus a small one over the toilet. We didn't find this necessary but it's like storing towels in plain sight. Also notice we chose a left offset sink.

Figure out what really needs to be stored in the bathroom and what can be moved to other areas of the house. The sink is not pictured but if the sink is a cabinet consider moving most storage under there and install pull out tiered shelving in cabinet to maximize space and make retrieval easier. A hand towel over the toilet gives me the heebie-jeebies yucky ! As noted in photos below move the light to above the mirror and have mirror same width as sink to feel more expansive. You could also do the whole space with a mirrored cabinet vs a mirror and cabinet so its one cohesive expanse doing a mirror and a cabinet may be choppy