Do you have any furniture you are wanting to put in there ? Does any of it have any colour ? I like the light floor colour , the rest let's just say it is a good neutral base . I'd start with a new front door -- a darker varnished ( glossy ) wooden door , or possibly even a blue to reference your outside . Then add colour via furniture ( or at least cushions ) , rugs , and artwork . Again , maybe several different blues would tie in with the floor , but also the exterior . If there are only blue exterior highlights , you may want to go for warmer colour inside -- reds , yellows , oranges -- again , ties in with the floor . To this end , I can't understand why people go with 117 shades of grey and offwhite and wheat -- live a colourful life is my attitude , as long as it is themed haha !

I would guess it is some kind of expansion joint. To allow for expansion and contraction of subfloor.

i would remove the old flooring, then the window then add the new doors, new floors and then all the new architraves and skirting boards....the new opening is the least forgiving structurally and you'll need to have the new doors custom made and packed up to match the expected height of the new floor ..check with your window/door manufacturers for specific advice ..we have a stacker doors and window that is recessed into the floor and the new flooring continues over the bottom reveal but if you want a bifold or swinging door the height of the bottom of the door frame will need to be raised to match the height of the new flooring

The skirting boards in pic are a meranti type colour as are the doors and architraves. Hubby does NOT want to change the colour or even consider white. Besides carpet what type of flooring could we lay that would still tie in well?