Where are your power points? The vast radiator covers seems the natural centre of the room to refocus the interest, so reorganise around that point with TV flat on wall. Conversation sofas either side could balance the room and make it more intimate or placing an L/corner sofa parallel to the existing radiator could provide you with a frame if the access to other rooms fron the front door is through this room. Currently all furnishing are pushed into the corners. You can easily move some power points and sky points and conceal the cables. Reinstating a fire place if the chimney brest has been removed can be a big job. You will be restricted to an alternative heating source, so any solid fuel burner could be expensive without an existing flue/chimney. You would be looking at electric wall mounted solutions or faking it entirely with a mantel/fire/hearth, so this then becomes a styling question for you.

Love the catflap in your internal door! I see what you mean about the door opposite the fireplace but it can be overcome. How many people do you need to seat? This style of room lends itself to a tv watching area maybe with an L shaped sofa and a supplementary armchair and the other end could be two armchairs with a bookshelf or sideboard against the right hand wall. If the door really bothers you, you could change it to sliding as there is room. You might have to move it up or down slightly. With nearly 5m across you should have room for the sofa and the door to open behind it.

I like your style which I would describe as sophisticated Scandi. Very clean. The house you are buying is a little bit more cluttered but still charming. Since you seem to have nailed your style why not try and use it the way the current vendors have with the play room at the back. Ikea kallax's could form a convenient inexpensive divider although the shelving is very useful. We are on our 4th house. We always change the layout to suit us but 7 out of 10 times revert to the way the previous vendors used it. Some layouts are just very practical.

I always go for the two largest sofas I can fit in within right angles to each other. Preferably one opposite the window and one opposite the fireplace. The distance between the two front ends of the sofa doesnt actually need to be that wide to walk through, for instance i currently have 1.5m width opposite the door and 1.2m wide with a 50cm gap between the sofa’s front edges and my sofas are 1.1m deep.