If you wanted to really do something interesting, I would get rid of the max beam rule, the uni rule, the spin hoist height rule, the foiling rule.
And just keep the 18ft long, 165k min weight, 20sq up, 40 sq down and it has to be a catamaran.
I would go one further.
Keep the boat 5.50 mtr (18ft) long, 130kg min weight then allow 2.75mtr width and limit total sail area to 40 sq. mtr. Allow a designer to use that sail area any why he wants.
Only then will you see some true development, mostly in the rig as the designers will try to find to most optimial distribution of the sail area over all possible sails. THAT could be very interesting. Mostly because they can keep the spi luff length but transfer some spi area to other sails.
Ruling that the class will have "20sq up, 40 sq down" is fixating the rig to the current uni-rig setup, thus meaning the only development that will be done under your proposal is making the boat a little wider. That is not development in anyway. Besides, isn't 165 kg ready to sail weight a little heavy ?
Also as the boat is now, even with extra width it will never succesfully compete with the other designs in the major cat sailing scene. The 18HT design must try to find a new elan or accept being confined to the Alps as a class. I honestly believe this to be true, so I guess the basic question becomes of what the 18HT class want for the future. Afterall they can be perfectly happy in the Alps as they are now.
Wouter