To be more specific than "chess", I was thinking we could make a chess database program similar to Chessbase or Scid, with a web UI which could potentially reuse components of
lichess, particularly
https://github.com/ornicar/chessground. There are a few directions we could go with such an app, but more on that
later (er, tomorrow. too tired now). If we could make something usable that would be nice, but I think the goal would primarily be as Bryan says. In my case I'd like to learn more about web frontends, but in particular I'd like to explore the reflex/ghcjs space more, and maybe contribute some generally useful code to it (I have a couple things in mind that seem to be missing). Bryan suggested we use purescript, but the potential of Haskell everywhere seems more interesting to me. Also, while being able to compile to JS is great for easy distribution, I'm imagining the app primarily being used natively, so that it can e.g. use a natively running (chess) engine. Maybe that's a point in favor of reflex as well.
I know you've used reflex quite a bit Dave (or at least, you gave some great talks on it at past meetings), so if you'd be interested in contributing then a reflex app might be a good fit for our different strengths.
In any case, I've started creating a reflex project with a couple additional bits of infrastructure that don't come with it out of the box (e.g. support for assets, and compile-time prerendering via TH). It's a bit of a mess at the moment, but I'll upload it once I clean it up some.