Well, I'm more interested in using Haskell, Erlang or Javascript/Node.js

"Another MtGox" is not a good idea because trade volume will be split between the two markets. I'd rather work on a front-end/mashup to MtGox. For example: a better interface (I can see a lot of possible improvements), futures or leverage scheme for MtGox to attract traders ( http://en.wikipedia.org/wiki/Leverage_(finance) ).

Some improvements will require MtGox to expose an API, but many will not.

I also want to develop a board game site with ratings and BTC bets. I'm going to start from http://en.wikipedia.org/wiki/Gomoku