Would extend the countertop that other 20" even if you don't enlarge the vanity base cabinet -- you can always park a laundry hamper or trash can under there.

Options for the 42" x 26" space on the other side include:

A traditional 42" x <26" linen closet open to the bathroom;

A traditional <26" x42" linen closet open to the hallway, w/solid wall w/ bathrobe hanging hooks on the wall behind the open bathroom door;

Floor to ceiling built in linen storage unit:

https://www.pinterest.com/pin/44895327510622439/

Note<26": slightly less than 26" so there is plenty of room for the bathroom door to open all the way.

I like hiyaluvs suggestion to use it on the other side if it works. Another bedroom closet or hall closet even might be handier than in the bathroom.

Is it too late to put in a sliding door? That might help make it more accessible if you make it into a bathroom storage space. We have the same space only it’s about half as wide. We did drawers on the bottom and open shelving above. Very handy.

The first thing I would do is install a pocket door that would allow a double sink vanity and then the area you are tak\liking about could be awesome storage well organized and totally accessible.

Open shelves. But leave space at the bottom for your clothes hamper to fit. That’s what I did behind my bathroom door. Perfect.

Whatever you do, just consider what you’d do if you needed access to the shower plumbing from behind. For that reason alone, you might want to consider a free-standing piece of furniture, like a hutch, or an étagère.

Both bathrooms in our house have that layout, and both have a closet behind the bathroom door. It's true that both doors cannot be opened at once, but that's not a big deal.

The storage space is wonderful! In addition to bed and bath linens, we use ours for suitcases, cleaning supplies, toiletries (on the lower and middle shelves), and less-used things (like Christmas dishes) on the top shelves. Our house isn't very big, so utilizing this space for non-bathroom storage is huge for us.

since we were out of budget, we 86d our built in bench and went with crates with a cushion on the plumbing side wall. I love the "bench" great for applying lotion, dressing or doing self pedicures.

these are my inspirations with hidden storage

My free version. I used a changing pad cover on a bench cushion I already owned on top of college milk crates ;)

I'd put in a 2nd vanity (and did - similar layout; 2 different size vanities but identical otherwise. We prefer it to double sink single vanity - even though it wasn't on purpose (its just what fit)). But I much prefer a 2nd sink over storage (we don't store much, and have a hall closet that's sufficient).