Why not make it "open season" regarding design, weight, sail area, beam etc, but limit it to having an ISAF / pn number that is equal or within a very small tolerance. That way, a H14 sailor can chuck on big sails to compensate for being heavy, while it makes no sense to build an all-carbon boat down to very light weight, since all that means is a penalty in smaller sail area to bring the ISAf / PN number to within the tolerance. I like the "14ft X 14ft x 196sqf" rule proposed by someone earlier. WHo says it should only have 2 hulls ? Or that it can`t be 1 ? Just keep the numbers equal. That would be a REAL development formula class. Existing platforms could add racks if the 14ft beam proves to work. Quickly detachable for easy towing etc.
Make it the tinkerer`s class !

Oh, and hydrofoils too, please.

Cheers
STeve