Modest GHC Proposals
There are many many proposals to augment GHC (and Haskell) that would be valuable yet languish because they have not be documented / collected anywhere aside from persisting in the Mailing lists.
Proposals are suitable for this page if they do not require deep changes to GHC, though they may still be nontrivial, and which ghc-hq is likely to merge in when there is a strong community consensus and well-written patch is on hand.
1 Records and Modules
Yitzchak Gale's nested modules proposal http://www.haskell.org/pipermail/glasgow-haskell-users/2012-January/021591.html which would address one of the larger warts with the current module system and records while adding essentially no complexity to the GHC internals. (ie no changes would be needed to GHC beyond the parsing phase possibly, so an easy chagne to experiment with )