I'm a long time member but not much of a poster, but any mention of F612's always gets my attention. Like many here an F612 has always been my holy grail guitar since getting to play one a long time ago.
A little history, I worked for a large midwest Guild dealer through the 70's and 80's and special ordered an F512 in 1973 (picked it up at the factory). I remember the day I ordered it the price list actually had the F612 listed ($1,000) and I almost ordered one but just couldn't handle the higher cost. To this day I wonder if I would have ordered it would they even have been able to build me one or were they already out pf production. I've heard that they were built in a few very small batches. Since then I bought a 1972 F512 with the fingerboard figures, large size rosewood bridge with slotted diamonds, tortoise shell pick guard, kind of an F612 lite. The difference from 1972 to 1973 is drastic, both are very good guitars but very different.
The older I get the more I'm glad not to have the 18"body, 17" body seems to get a little bigger every year. If Cordoba had any plans of a custom 12-string, the following is my idea of the way to do it:
F512 Custom (or call it an F612)
17" rosewood body, flat two-piece back
Bound ebony FB with figures but do a nice job on the inlay (not like the originals)
Checkerboard purfling on top, sound hole and peghead
Large rosewood bridge with pearl inlay on both sides
Tortoise shell pickguard
Full size gold plated Grover machines a must
Basically F612 specs in a 17" body. With the new bracing I would think it could come close to the deep bass rumble that the F612's had without having the big body. Maybe, while they're at it, an F312 version (with mini Grovers) would be fun.
There you go, my two cents. Sorry for the long read, hoping Cordoba reads this and thinks "hey, we could actually do that pretty easily" We can at least hope.