Just a few thoughts here, prefaced with a disclaimer:
Disclaimer: I know very little about touring specific bikes, other than what I've seen/read online. I owned a Nishiki Cresta tourer for a while, but it was a smidge too small. I've never toured proper, just many day trips, no overnighters. I spent 7 years as a bike mechanic from Colorado to California, so I've seen pretty much everything that can go wrong with a bike. That said, here are a few of my random thoughts:
Dedicated touring bikes appear to be the perfect machine for those of us who have no delusions about our being Tour De France winners. Long, stable, durable, versatile, and these days grossly overpriced for a boutique version. Someone said it well in a previous post: Most of us are not going to travel the world, rather, we'll be sticking to paved roads and maybe gravel.
With that in mind, it seems to me that (and I may be wayyy off here) early MTB's would be ideal. I have a Peugeot Canyon Express from 1985 that is more tourer than MTB. Dual eyelets front and rear, lowrider mounts on the fork, three bottle braze-ons, and a real live pump peg behind the seat tube. Looooonnnnngggg chainstays (18"!!!) and laid back geometry spell long days in the saddle. Fully lugged, double butted Ishiwata cromo frame and fork round out the package. Best of all, I paid 40 bucks for it at the local thrift store.
I have a full blown custom Curtlo Epic Mountaineer that cost me nearly 3K by the time I was finished with it, and the Peugeot gets out of the garage much more often than the Curtlo.
Any advice I can give is probably null and void due to my lack of experience, but it seems to me that the real requirements of a touring bike/frame would be long c-stays, laid back geometry, and enough braze-ons to allow maximum flexibility in packing/loading.
MTB, Hybrid, or dedicated touring bike becomes irrelevant if these fundamental requirements are met. And many early mtbs are very very nice if you can get past the "old tech" stigma.
Just my .02.