• The 1.75u key in the spacebars kit should be replaced with a 1.25u convex space key.
Explanation: 1.75u space is only used on three 40% keyboards in total, whereas 1.25u is useful not just for Alice layouts, but also for common split spacebar configurations (2.75u+1.25u+2.25u) on many different layouts.
• Since there is what appears to be basic ortho support in the base kit (thanks to the 1u novelties), you should probably include 2× 1u space in the spacebars kit as well, for full grid ortho layouts.
• Finally, consider adding a R3 1.25u Fn or Tab key to base for basic staggered 40% support.
• I would also like to see R3 PgUp, R4 PgDn added to base, but I understand if you decide against this because of the presence of 1u novelties.
However, it would be nice to see a R1 novelty added as well, as this would let 65% users cover their entire right column with novelties.