As boring as it may be, a simple beige and white set could be hugely valuable as an "infill" thing.
If you want to put decent caps on an AT101W, a Ducky 1008XM, a Sprit (did he ever release the ALPS boards?), Kingsaver, or GH122, you're almost certainly going to go mad trying to find a few modifiers.
I could see an inexpensive "super-Tsangan kit" being quite useful:
Bottom rows: a full set of 1.25x modifiers, as well as 1.5-1-1.5 1.5-1-1.5
ANSI 2.25x enter. 1.5x |\.
2.0x backspace
Potentially Kingsaver-sized right column keys
LED and non-LED lock keys
Potentially a function row for people scavenging from old boards with only ten function keys
Take that set, combine it with cheap caps from a Focus 2001, an Acer 6311, or some a random Chicony board, and you can cover a LOT of more modern layout needs.
Yes, not everyone would need every key, but I think this has to be a "You subsidize the parts I want, and in exchange I subsidize the ones you want" situation.