Disallowing new designs and therefore innovation is short sighted. Also, the forum name is "classic pattern" and not "classic pattern airplane designs".
If you have a new design then it is no longer a classic but a new design. Now perhaps you mean't modifications to existing designs?
But even then it would not be classic pattern. Classic hot rod pattern perhaps. If you like that then start your own group. SPA and CPA don't have to conform.