Please consider adding Colevrak homing keys. The Alphas kit already has two required keycaps: non-homing F & J.
Here are three ideas to complete the coverage:
1) Low cost = add two blank homing keys to the alphas kit. This would support any alt layout, although without legends it's not ideal.
2) Standard = add homing U, H, T, and N to Alphas, and add 1.75u Backspace to TKL mods. These five keycaps support at least nine layouts: Dvorak, Colemak, Colemak DH, ColemaQ, Asset, Workman, Niro, Norman, and QWPR.
3) Deluxe = add an Eight-Finger Homing child kit. Fifteen keycaps would cover QWERTY plus all the layouts listed above: homing U, H, T, N, A, E, S, I, D, O, R, K, L, and semicolon/colon, as well as 1.75u Backspace.
One more thought: add the five standard keycaps in 2) to the ISO kit. Appealing to a wider audience, the resulting ISO/Colevrak kit might sell better than ISO alone. Especially if all the vendors offered it.