I'm not necessarily opposed to this idea, but I'd like to point out that it can have a negative impact on readability of an individual module, since you can't tell which Prelude is being used. This is the same argument used for putting LANGUAGE pragmas in a modules instead of listing them in a cabal file. I think in the case of an alternate Prelude, the argument is stronger, since language extensions often don't change the meaning of code, but instead allow previously invalid code to be valid.