As my fixed gear frame hits 30 years old, I start to think about replacing it. The Surly Cross Check has very similar geometry to the old Super Course, but with a higher bottom bracket and perhaps slightly sportier rear stays. The frames in the basement would do for a knock-together beater for pure urban assault, but I put in longer miles on open roads up here.
The Traveler's Check offers at least two bikes in one, since I could build it with gears or without, depending on the destination. For instance, for a quick plane trip to many urban -- or even rural -- locales, I could configure it as a fixie for simple assembly and disassembly. For an air leg en route to a more major tour, it could have the full treatment with gears and more rackage. If the area offered longer road rides or terrain that might be too much work with a fixed gear, build it up with a few gears and minimal cargo capacity.
Ah the joys of friction shifting.
The next step beyond mere S and S disassembly would be mix and match front and rear frame sections. Reconfigure the frame geometry for different venues! Snap in the rear section with long stays for loaded touring, mid-length for general riding and short and snappy for agility.
Yes, I'm getting crazy here. Front end treatments have to be compatible with the ride feel of the back end, so you probably end up with just a collection of coupled frames in your desired types. But starting from the basic Cross Check geometry and dimensions, you could make minor tweaks to fine tune the platform even more to different missions. You can see how I might start down that path.
As the Next Frame to Buy, the Traveler's Check takes the adaptable Cross Check to new heights. That much is enough for now.