As i said in my comment on the review of the Western Flyer, there's one big problem (to my mind) with the design of the Western Flyer: either (a) you have to pack the sling strap away in the back outside compartment of the bag, when you're not using it, or (b) you have to leave that compartment partially unzipped so as to get the strap out, since it's sewn in inside the compartment. As someone who might use that strap once in ten times when taking the bag somewhere, (a) is the major disadvantage for me, but for someone who actually uses it, (b) would be a real downer, too.
My suggestion would be to offer a bag just like this, only without the sling strap altogether. Alternatively, you could make the strap detachable, and attach it at the top with a pair of recessed fittings (comparable to the fitting at the bottom of the strap) entirely external to the compartment on the back of the bag.
Just a thought. A compact travel bag with room for a couple of days clothes as well as papers, miscellaneous stuff, and a laptop (like my new MacBook Air) would be fantastic. That's what I thought I was getting in the WF, but alas, it didn't quite live up to the promise. At least not for me.