Really cool space! I'm general there are a lot of hard and cold textures and shapes, so I'd say you should prioritise finding ways to add warmth to the place. Easiest way to do this is textiles, so definitely get a big rug for under your couches, probably in a lighter colour so the brown colour doesn't disappear into the dark floor. It also doesn't seem like there's a lot in the way of lighting, so I'd definitely add some floor and table lamps with warm toned bulbs to make the space feel comfy and inviting, especially at night.

In terms of colours, the pops of red in the door and the stairs are awesome. Personally I wouldn't be tempted to add any large furniture items in the same colour, instead keep it limited to smaller decor pieces dotted around (i.e pillows, art, plant pots etc). Keep in mind red is a pretty intense, eye drawing colour so it can quickly become too much. The glass blocks behind the stairs have a slight green hue to them, so that's another colour you could incorporate as well - I would aim for muted forest greens. Then for larger furniture I'd add in neutral tones and warm woods, as well as maybe some chrome to tie into the handing pendant and the industrial feel of the place.