Why not adopt the old B-Class Rules and add a 25 sqm spinnaker?

Max overall length of 20'
Max overall beam of 10'
Max overall sail area (including mast) same as current International T.

You might want to adopt some form of weight adjustment to equalize boat weight/crew weight that allowed heavy weight crews to sail on lighter boats (reduce boat weight by 1/2 lb. for each extra lb. of crew weight) to make the sailing more equal, just like F18 does. Just some thoughts that would stimulate boat development and allow existing International T's, M20s, Eagle 20s to compete boat for boat.