day bed is a pretty cool idea. Nice to lounge in. Cream color with brown cover pillows to match wooden desk. (and some dark red thrown in)
if you have a free wall, you could build-in shelving for games / books / dvds / figurines etc. An allocated section for everything.
Put back lighting on the shelves so has a subtle glow when you play at night. Will give you some light in the room at least. (not good to play in dark) Try use matching wood color to your desk.
depending on size of room, you could get one of those room dividers to separate the desk and the couch area.
I've got one in my room, a nice wooden carved one, so my pc den is behind it - with book shelf / another desk on the other side. (still need to finish my own pc room, it's just a dump atm)
got a dark ancient oak wooden flooring put in somewhat recently, so getting in furniture to compliment it. Still need to finish painting.
paint the wall one darker color (dark red?) and the rest a lighter color (neatral / beige). Will give the room some character. You can also get some funky wall sticker decals, saw some gaming ones somewhere, and then you get ones with a black city scape of buildings etc. makes a awesome feature on your wall without looking tacky. You can use them with posters in frames, and the design stickers joining the frames up.
if you have empty corners - either put in a nice floor freestanding light that will blend in with the decor, or those ottomans? the poof things. I don't know the name. Some you get longer ones where you can open up and store things in - go for a leather version (or fake leather) to go with the rest of the wooden theme. (leather, wood and subtle yellow lighting looks great, especially when add a dash of dark red)
Make sure you get a desk lamp for your desk. using the main lights can be too bright to play games on.
Also get a comforter / throw blanket for the day bed, even dudes get cold you know. Great for when watching sport
Seeming you have the sink - make use of it. put a cabinet / shelf next to it with a coffee machine, maybe a mini fridge. very simple but make the cupboard in a matching color to your desk.
put shelves in places with any figurines / statues / hell sports tokens if you got em, anything you like or want on display.
can be a tower of beercans, or different bottles.