I am not against changing the Prelude! But it sure would be nice if
-XHaskell98 gave me a Haskell 98 Prelude and -XHaskell2010 gave me a
Haskell 2010 Prelude, both of which could be used with external packages
that themselves used the more modern Prelude.

This sounds like quite a herculean task. Changes in base type-classes would wreak havoc between preludes from differing timelines.