Sakkit bicycles came were offered with the option for a security 'plug' built into the seattube, a waterproof sealed compartment accessible from a fitting in the seattube between the derailleur and the bottom bracket.

IF I were designing a custom touring bike from the ground up for my specs, I would get the fitting for this - have seen it at framebuilder sites that offer lugs, etc... and ask it be installed in your frameset. I suspect you could have it installed right at the derailleur clamp area so it would blend right in.

you never know when you're going to need to smuggle some diamonds across a boarder

An integrated rear rack with 15 inch deck might be nice.

decks and fittings for water cans of some large sort if you are travelling third world as well. a simple inverted T configuration with 3" angle rail at the bottom with a strut leading to a strapover to secure an edged water container on the rail of the T would be sufficient.

Water carrying on the front fork is nice. maybe integrate a watercan hauling system with a lowrider rack, integrated and welded to the fork.

I believe there was a fairly well known, record setting, knighted British cyclotourist that used integrated jerrycans or something like this, he recently passed, RIP, and I've forgotten his name, it escapes me.