If they wanted to incorporate Math.NET Numerics (which is pretty good, I have used it before for a lot of stuff) into the standard library alongside the existing RNG architecture, I'd be down for that, but I expect the same seeds to play for me the same sequences everywhere because if they don't simulations, games, etc. will break.

Eventually you can deprecate the existing System.Random and turn its implementation into (using Math.NET Numerics terminology) a separate RandomSource, and eventually remove System.Random, but you can't change the contract of System.Random.