I don't plan on buying another camera for a long time. My approach is very pragmatic, and I only feel like I need to have one camera.
My Hasselblad 500 and three lenses was a birthday gift from my parents. How lucky is that? I don't feel like I need that camera, but it sure is nice to have, and it is such a joy to use. I could probably stand getting another body for it, and a back-up couple of lenses, but I'm very happy with what I've got.
The Leica M2 was offered to me a couple of years ago at a price I could not turn down. I can't fault that camera, and in addition to the Hasselblad it's hard to find a reason to actually need another camera. One stellar 6x6 and one stellar 35mm.
So, why I have kept my 35mm Pentax gear, containing a couple of KX bodies, six lenses, a Spotmatic with a 58mm Helios, a ZeroImage 2000 pinhole, a 5x7 Century, and a Rollei 35SE I have no idea. But I guess it's fun to shoot macro with the Pentax, the Spotmatic is just a cool camera, the little Rollei is a superb little pocket sized camera, and pinhole and 5x7 is entertaining and used to shake it up a little.
I guess my criteria for buying another camera would be that it would have to fill a function my existing cameras can't. I can't see what that would possibly be, but perhaps some day I'd get really interested in interior architecture, for example, needing a 4x5 with crazy movements. That sort of thing.