I suspect the market for something like this is pretty limited, but I'll throw it out there...
I have a 17" Powerbook. When I replace it, I'll likely get a 17" Macbook Pro. This severely limits my choices when it comes to Tom Bihn bags--I think I'm limited to the backpacks, the Super Ego, and the Empire Builder. I'm currently the owner of a Brain Bag and a Super Ego, a Monolith and a horizontal Brain Cell.
On overnight or extended trips, the Brain Bag is great--it fits everything I could possibly need. The Super Ego is great for days when I need to take a bunch of stuff to work with me (A coat, an umbrella, a change of shoes, etc.) But for just 'going out'--or the summertime commute when I'll be taking substantially less with me--I just don't need that much space. I'd like something that would just about fit my laptop (in a Soft Cell), its AC adapter, my phone, my keys, and my wallet, and I'd like it to be messenger-bag style, since it's easier to access stuff. I've debated getting an Imago as a running-around bag, since I really like the way the cork looks on them, but have yet to convince myself it'll get frequently used since my computer won't fit in it.
(I'd also like to see TB come up with some sort of laundry bag that doesn't fall apart after a couple months of use, but I suspect that's even more of a niche market.)