My precious 15-year-old commuter mountain bike is ready for the pasture. I don't have a lot of storage space, and I'm longing for one bike that will do it all: commutes along with moderate road rides and off-road use (no jumping).
If I take a nice, light aluminum touring frame that's built for 26" wheels—a Dutch/Asian Koga-Miyata or something cheaper if I can find it)—put MTB parts and flat bars (built up to easily switch out with drop bars) and lockable front suspension on it, will it be strong and agile enough? Or will it be limited to mild fireroads? I'm 180 and somewhat hard on bikes—I've pringled/cracked a few good rims on moderate rides.
—Haul 'em like Gollum
I too have dreams of an elegant omnivore for trekking not only to Powell Butte or Forest Park, but through them as well. There's a lot to be said for staying SUV-free and riding from your front door and back—even when dirt is your destination.
I think what you have in mind would work, but I have reservations. The Koga-Miyata seems to have a long back end (typical in touring/trekking bikes), which might make it hard to find the right weight distribution for traction on steep climbs. Also it doesn't appear to be suspension-corrected; with a shock fork the front end will be jacked up higher, which will decrease the head-tube angle and make the steering seem sluggish. It might handle like primordial mountain bikes even older than your retired commuter: fine downhill, limited otherwise.
Intead, might I suggest as a base the Surly Karate Monkey? It's a bit heavier than what you had in mind, and it's in short supply right now—but it has supreme versatility. It's intended to be built around 700C wheels (the standard size for higher-quality road bikes)—either as a 29er mountain bike or as a cyclocross-style bike—but keep in mind that if you use disc brakes, you could swap easily between 26-inch and 700C as conditions dictate. Besides, you can get some pretty stout 700C rims these days.
One other note: While dabbling in adventure racing over the previous two summers, I found I could use a drop bar very effectively on single-track, as long as it was extra-wide and extra-high. I have a freak-of-nature, discontinued 50 cm anatomic bar, but other options exist.
If we ever get these bikes built, we should go for a ride. Any other all-rounders out there?