Open plan living/kitchen/dining layout help!

[Somanyusernamesunavailable]

This is my first ever post!
We need help please, I welcome any opinions as we're stuck on the new layout of the back area of our house. We are getting a double side extension, to move family bathroom upstairs and to put utility room and office downstairs in new side extension.
In the picture, the walls of the 'workroom' and store and bathroom are all coming down to form a large open space. But we're struggling to decide what to put where.
Maybe a galley kitchen in the longer thinner bit? Could have a peninsula with seating at the social end to be close to dining table.
Or do we put sofas & tv etc down the skinny end instead? We are keeping the front living room as a second lounge. Will be a decent size and walled off from the open plan living space, as part of the side extension is a new hallway area leading to the back. The stairs might also move into extension.
Hope that all makes sense! Does anyone have any wise ideas?
The picture attached is our current layout.

I can't see what you're doing - are you building out at the back so that the right hand wall of the bathroom / store etc is in line with the right hand wall of the kitchen (so one big rectangle)? Or do you mean you're building out, but that new part is going to be the office / utilty so you're still left with the long skinny bit on the left and don't know how best to use it?

you'll need to post a floor plan with the footprint of the new building otherwise there isn't enough in your post to comment on properly.

Is the dotted red line the new outline?

If you are moving the bathroom upstairs will you need to relocate the downstairs loo? If it were my house I think I’d keep the workroom but turn it into a loo/utility. Id also keep that bit opposite the workroom as storage.