At the moment the GHC API is a sort of poor relation in the haskell world, where it could be a significantly useful resource for the growing list of haskell tool providers.
Based on my experiences with HaRe, I have started putting together a wish list of features I would like to see in it, which is here
https://github.com/fpco/haskell-ide/wiki/GHC-API
I welcome feedback / discussion on this.