I tend to have a stereotype of Haskell as the most "advanced" (high-level, sophisticated, powerful, abstract) language. One which has lots of weird features that, if I could grasp them, would make me a super-hero programmer.
But what comes after Haskell? What languages and new kinds of abstraction are being invented now in academia which make even Haskell look like Blub?
I think you're looking at this the wrong way. Learning Haskell is a great way to stretch by pushing yourself to work in a new paradigm. Certainly, it's unfamiliar if you're used to procedural programming.
However I wouldn't put these things on a hierarchy, as tempting as it can be. I know that a bit of online reading will give us all a clearly defined hierarchy of coolness: C++ programmer more hardcore than java, ruby cooler than python, yadda yadda yadda. My suggestion would be to pick something either intellectually interesting or liable to be useful in your next gig(although haskell, which I'm currently messing around with as well doesn't seem likely to be) and learn that. There are probably 50 good suggestions on the next cool thing that would be great for you to learn, the best of these for you will be the one you're actually interested in.
There are lots and lots of niche languages, however mostly they express a few basic paradigms. It's probably better to get a handle on the paradigms than to worry about the next best thing. After all ,lisp, a great language to learn for a lot of reasons, has been around now for about 50 years.
As for practical functional languages, I've seen just about 0 for Haskell, a tiny,tiny bit for Erlang, but near enough to 0 not to matter.