From the contributions so far it appears that the waterballast system is indeed the key feature for new boats.
Then, there is a divide between a smaller ca. 16' open boat vs. a cruiser (cabin) version of an existing 20' - 22' boat.
Swallowboats already have quite a few smaller open boats, which may not have waterballast, but which are nevertheless very stable and sailing very well. (We just had a direct comparison between the Storm 17 and both Raiders in the Morbihan, and the Storm pulled through in all conditions including a choppy F7 without fail.)
So, my thought would be that a new cabin boat would improve the overall spread of the range of Swallowboats best.
22' would be my favourite length, because you want a bit of comfort insid a cabin AND still have enough cockpit length left for the full crew to sit in the open, and a dog.
The SeaRaider really isn't a good substrate, it is purpose designed for speed so it has too little beam to offer real cabin comfort.
Having seen and sailed the BayRaider last week, I think she has everything for an excellent cabin version too, except maybe 2 extra feet in length. So, may I suggest to think about a Bayraider Cruiser, which would have 2 extra feet of length built into the hull? She would compare remarkably well against the Drifter 22 and the Cornish boats, which all suffer from trailering overweight, you cannot even step their masts singlehanded.
Claus