Without going off the deep-end with modularity, it might be nice to have a way of incrementally building the library. For instance to specify which types are included when building in the grunt cli.

Not sure how this would look, or whether including a separately built directory for grabbing absolute require paths might be preferable, e.g..

var Map = require('immutable/type/map')

You'd lose out a bit on the bytes shaved by compiling the library as a single file, but I'd imagine omitting entire types would make up for it depending on the case.