Disclosure: the full featured game will be free, and I'll be releasing my art creative commons. However there will be a paid version where I don't bug you to buy the paid version (pay once for full).
My design problem is this: I have about 50 variants and plan on adding hundreds more (since it is easy). What I cannot do is bombard new players with too many choices. Also, I need to introduce new players to new pieces gradually instead of all at once.
A solution I'm considering: some kind of adventure, map, or quest mode where new players are guided to play chess variants which start very close to classic chess (with maybe one new piece, or a weird shaped board). As you progress, the rules for the levels you unlock stray farther from classic chess. It's easier to play a variant with 3 new pieces if you've already played with 2 of them. I haven't decided on UI or movement yet, but I'm starting to think something like the overworld in Super Mario World:
I'm going on a two week vacation to Vancouver tomorrow, however I plan on releasing a free open beta to this community (and others) soon! Again - feel free to give any design suggestions or feedback, especially links to games which you think have solved a similar problem. I'm happy to answer any questions.
Cheers!