I think a lot of interest would be shown in replica cases, tbh. Face it, no one was buying the C-USA C64's for low cost or sheer quality, she was a nostalgia factor only in my books. At least it was something unique with a modicum of innovation,
even if it was done by some guy on mini-itx.com 10 years ago.
As for which case to recreate, that's a real diceroll. Some people like me have always hated all in one Amiga's like the A1200, while others just love 'em. I simply dislike built in keyboards, though. On the flipside, some people dislike the big box Amiga models due to the real estate they take up. Just a matter of preference, really. The A3000/A4000 T are a much beloved model as well, but I doubt many people would want a case taking up that much real estate in this day and age.
I for one would absolutely LOVE an A1000-like case that I could slam a Mini ITX board like my SAM 440 into, and perhaps a good quality keyboard that matches said case perfectly to complete the vibe. IMHO, the A1000 was *the* classic Amiga model, and would still look real sharp on any desk. The good thing about modern PC gear is the ATX standards as far as backplanes and such go, I guess.
Legality is a dicey issue though if there are any plans of molding in badging similar to the original, but if C-USA can sticker stuff up, it's not foolish to think some independent party could print/mold decals or badging to put on Amiga cloned cases
All you'd be selling was an "Amiga" inspired case design, really.
I figure there would be significant interest in something like this, enough to get the ball rolling via a Kickstarter fund anyways. Would definitely be something I would support financially if it was good quality.