Our den is an awkward spot and we wanted to have a usable double bed as well as desk space for two people, but putting in two desks and a futon made it really cluttered and took up a lot of space, plus we were always having to move one of the desks in order to use the futon as a bed. So we combined all three items!
I changed the dimensions of the loft to accomodate the futon frame (I actually just dropped the futon bed support into the top of the loft) and then I used a long counter I already had as a desktop. The desk is sitting on large shelf brackets that are attached to the uprights of the loft, and then at the front there are shelves to hold up the front part of the desk.
It's ideal because most of the time we are not both working and so we can take advantage of the other person's space, but if we are both there there is enough room as well.
The plan I used below is awesome because everything is really firm but you can still take apart the key parts to get it out of the room. So I built it outside but put it together in the room itself.
We call it the sky bed and my friends love it. I also ditched the ladder and built the shelving unit you see that is both a ladder and shelves, and takes advantage of the shape of the room.