There is no HTML 6. HTML5 is a living standard though the W3C publish point releases on the spec, this release is more about versioning the spec document at certain points of completeness than being a version of the language or feature set.
Also of note is that the web platform as a whole is a combination of various specs and more than just HTML.. Each of these is discussed separately and while there is some cross over between members and contributors they are relatively separate.
With how the web evolves the idea of finite versions for the web don't really make sense with features getting added basically as they are done and differing browsers implementing them in different orders, you can realistically only talk about specific features.
Standards are decided by a wide variety of people with various positions. The W3C itself is a key player but direct involvement of the browser vendors outside the W3C play a big role. Some features started in the community and are championed up to the W3C. Decisions ultimately come down to vendors as an unimplemented spec isn't worth the bytes it's stored on. Some vendors play more with the community than others (eg Microsoft has come a long way round from the IE6 days whereas Apple have gone the other way).