Waiting for this to drop. a red Enter key perhaps? Even as extra or part of any kit, to capture the flag colors.
That's why we have the Japanese flag novelties. I'd want to keep the monochromatic feel that the core has, adding more red accents to the novelty kit would be too much.
In order for the 40's kit to support Preonic there needs to be an R1 1u Backspace. I have seen some GMK kits that provide one in the main kit (don't know if some other boards use a 1u Backspace), but you do not include it in the main kit or in the 40's kit.
I've considered in the past adding it but then I start thinking about it and it doesn't make much sense to do it. Here is my reasoning: The OLKB keyboards have that philosophy that they were designed in a way that your fingers would hardly move away from the homerow, and that's how the planck was born. Your fingers just move one row up and down and you should be able to reach Enter, Esc, Tab, Backspace, Alt, Ctrl, hardly moving your hands. However, when the Preonic came, a 5th row was added, which solves one of the issues the Planck had (people not getting used to accesing numbers using lower and raise) but its default layout it's not consistent with the OLKB principle, because instead of keeping the Backspace in the same exact spot as the Planck (which very easy to reach, and it's very similar to what we could find with an HHKB layout) an essential key is moved away from the fingers making it harder to reach. Since the Preonic is easily programmable, and there are decent alternatives (such as delete) and the previous reasoning, I opted not to add either a R1 1.00U backspace to the Core kit or the 40% kit.