• Switch random package to depend on splitmix, instead of the other way round. Get rid of the internal StdGen implementation in favor of SMGen (i.e. type StdGen = SMGen).
• Improve the implementation of each individual instance of Random
• Make it possible for implementers of custom generators to override default implementations of individual primitive types (i.e. customizable random function).
• Add an interface to random package for stateful generators, which could be used by mwc-random and others alike.

See https://alexey.kuleshevi.ch/blog/2019/12/21/random-benchmarks/ and https://github.com/lehins/haskell-benchmarks.