I think that it could be very interesting to extend the playground to support other typecheckers (e.g. Pyre or Pytype). It is worth noticing that type checkers can have different type inference.
As far as expanding to pylint & others, I keep a (very old) fork of mypy-playground that I use for CrossHair, on the web here. Not surprisingly, I'd be more than happy to deprecate it into a more generic web tool for static analyzers!
I think that it could be very interesting to extend the playground to support other typecheckers (e.g. Pyre or Pytype). It is worth noticing that type checkers can have different type inference.