Lots of the guys who have youtube swing bike videos are doing multi speed. Although I'm not sure if the extra speeds can really be used much with the bike in swing mode.
I'm planning a SB build, and like you , I have an original. I'm thinking a larger size myself, as the one in my avatar is a bit on the small size and I have the sissy bar up all the way, which makes thing a bit floppy back there.
I'm also considering using the separate pivot design instead of the pivot at the seat post, hopefully to get the bike a little easier for beginners to ride and maybe eliminate the need for springs.