Well I appreciate the clarified intent of the thread but, after reading the whole thing, I can't help but echo a couple of things before I sit back and watch...
Meters: I'm generally in favor of them. seems to me that the Pre is the place were you want the finest control of your sound and so, that's where you'll want to measure it, yes?
Inputs: you can't please all the people all the time, of course. Me, I'm all for standard XLR's but, your custom multi-pin y-cable notion intrigues me. I suppose that if you made a cable for to order for each pre, that would work, but, I'd hope you'd publish the pin-out so that hackers could also make their own down the line (I don't see why you wouldn't.)
Power: yeah, lots of folks carry 9v batts around and 9v would be ideal for them but, if 12v is what it takes, then 12v is what it takes. Still beats all those terrific pre-amps that requires A/C power.
(extra) Inputs: It would be cool to allow a matrix circuit for say, an additional 2 channels line level input. But maybe that'd be best saved for the next box. Get one thing done right before you add to it, right?
Size: let function dictate size within reason. I carry a laptop half the time, who am I to tell someone to make things tiny and lightweight.
Hell, after making all those little stealth-able boxes, your eyes will probably thank you for working on something a little bigger.