Maybe if you stuck to one colour scheme such as white and purple or grey and lime green, colours that compliment each other. You could have a cabinet on the larger side with some family photos or a mirror above it and maybe a single colour on the top half of the wall and a complimentary wallpaper on the bottom half. You could go for wood on the floor or a simple soft carpet but it might get muddy. You could also have a bench with storage underneath for shoes and bags, but try get a colour that matched the walls. You could also paint the banisters if they don't match the rest of the room.

If you don't want to make the wall itself straight, maybe consider playing with some deep shelves ABOVE the TV, that could go from the diagonal wall to the straight line where you want to put the TV. Then behind the TV you could have a vertical panel of the same colour/material of the shelves (or matching the colour of the wall) to connect the TV stand to the first shelf and hide the awkward wall. To avoid having empty space right beside the door you could put a narrow shelving unit (with a back panel, not an open one) between the TV stand/panel and the straight wall (or beside it if it's short) that goes from floor to first shelf, again I'm imagining same colour/material of the rest so it binds seamlessly rather than looking like patchwork to fill the space. It's a little complicated to explain but I hope it makes sense!

A great uplift for an old house great

Hi, I think any of those 3 could look good, with white elsewhere as a contrast and bringing in some bright warm colours with cushions, throws etc. We repainted our house last year and used Farrow and Ball colours - Elephants Breath is very popular and looks great in our living room, we used their darker greys in our sitting room and hall. You can get Dulux to mix FandB colours so it works out cheaper if you like. I was afraid of it being cold or dark but love it!