The "Security Considerations" section talks about the possible need to
replace SHA2 with other algorithms, but the name "sha2" is baked into
the kex method name. Should the hash algorithm be a parameter too?
That's an option, although one that I don't think is worth pursuing at
this point. The SHA-3 competition is a long way from being done, so
SHA-2 is the gold standard right now. If there's a desire to see SHA-3
adopted in SSH, it would not be hard to create a short RFC indicating
that SHA-3 could be substituted fro SHA-2, and this draft, while not
explicitly saying how to do it, is very suggestive.