I really enjoy my single speed. Tried the fixed angle but after a couple of tries a few years apart just came to the conclusion that my brain is hardwired to the "coast" setting.

Mwu, you mention "novelty" in your first posting. And really that's what this is all about to a large extent. The "sensible" option would be a cyclocross or touring bike, no question about that at all. But why does every choice need to be the sensible one? What ever happened to FUN as part of the equation?

Riding a mountain bike with road slicks can add a lot of fun to the ride since with the right tire option you can ride through spots where a road or cyclocross bike may not go. Like down stairs or through that loose park trail to get to where you need to. Or even offer up some jumping off low barriers or whatever. Similarly a fixie has it's own intrest.

But if you're looking for a part time trail bike then I'd have to say that the mountain bike sounds like the way to go at this point. With some careful quick change mods you can set it up to go from a fender and rack commuter to a trail bike in a few minutes and then back again. A second wheelset with knobbies would speed this up as well. And if you go this route I'd definetly recomend discs. Having had both I'm partial to hydraulics with dual piston calipers. They are less fussy to set than the single moving pad mechanical or cheaper hydraulic types and really do feel nicer and more crisp. Rumours of more maintanence are just that. They require DIFFERENT skills but once learned they don't need as much or frequent attention as mechanical setups.