it's an investment... ask a plumber to take a look and advise.

And a challenge…and without measurements it is hard to be sure…main concern would be exterior venting and soundproofing ….a longish, but narrow floor plan… with the door in the center with toilet and sink at each end….against the the staircase wall would blend in best

Would an under-the-staircase be a possibility if the stairs going to the basement are under the stairs going to the second floor?

That may be possible but usually dramatically increases cost as moving doors ,walls, may become necessary…and the added
Little triangle of space adds a cramped feel to an already small space…

If that odd little closet spot ends up in the kitchen..it could be handy storage for big lightweight things like paper towels..toilet paper…etc…

I wish it extended to the kitchen but it’s just a small space.

Did you eliminate the bathroom in the now kitchen below the blue line ? In that case can you use the plumbing and add a toilet / powdered room outside ? And somehow incorporate it to the house ? Ask a builder / plumber / architect

A coat closet perhaps?

There is no coat closet.

A-coat closet … In the tiny space..?

No coat closet. Just a Harry Potter space.

Living room, dining room, kitchen in a row.

I’m confused. Isn’t there a powder room in the floor plan you attached?

Sorry. I drew the line a little short. This is not from our house. Just the idea of the set-up. There is no powder room.