I too second the inclusion of Alt Gr keys. I do not care whether it's in the base kit (that's already a no from what I understand), in any other kit or as a standalone kit. It would just be so damn nice to finally be able to get a GMK keyset with the Alt Gr keys there.
IMHO, please consider including different sizes: 1.25u Alt Gr for standard layouts, 1.5u for HHKB/WKL and 1u for ortho/ergo/40%/60%-with-arrows layouts. Personally I could do without the 1.5u one, but I'm sure it's popular enough amongst others.
I'd also like Alt Gr keys, but it's not really a deal breaker for me.
Speaking of deal breakers...
I've noticed that GMK is slowly changing the legends on some keycaps: The G from the recent hiragana keys is different, and now on Burgundy R2 I've also noticed that the legends of the symbols on @2 and #3 are clearly different from the old ones. Are they going to change any more of them?
While I understand that most people don't care about this, I personally think that some of the handcrafted symbols are kinda iconic, like the large ^ over the 6 or the stretched #, but GMK seems to be slowly replacing them all.
I think this should be a concern also for fans of old/vintage sets, but I don't really know, maybe I am the only one that cares about this