There are so so so many options but if it's a forever home I'd employ an architect.
It depends on what you need tbh. I would, in a dream world, want an extra bedroom, the bathroom upstairs. And at least on other reception room.
I'd knock in the kitchen, dining room and store and build out so that you have one large family kitchen room (and an architect will be able to tell you how to do this sympathetically and to keep the fire place). I'd probably want some kind of utility too and maybe a pantry.
Please please get an architect, my cousin didn't and she just spent a lot of money of something that doesn't quite work. Long term it'll be worth the money.