Here is the crazy idea; instead of having a vim plugin to "rule-them-all", why you don't join your efforts and create a plugin which does only indentation for Haskell code?
This way, we could have a modular stack and be free to use whatever plugins work for syntax highlighting / unicode syntactic sugar / whatever, but still have a "de facto" plugin for indenting haskell code in Vim in a smart way :)
A.-