Like the thread title says, what would be the next bike you would buy? Would it replace something or would it be part of the N+1 strategy?
The next bike I'd like to get is a Surly Karate Monkey single speed with disk brakes, fenders, rack, lights, etc. for a bullet proof all weather commuter. I never change gears on my Marin when I go to and from work (maybe 100' of elevation change over 13 miles), so going to a SS wouldn't be a huge adjustment. I'd still keep the Marin though for when I go on rec rides to the hills.
How about you? What do you want? Sky is the limit.