Alts starting at lvl 55.

There are no current plans for this for any class but of course the existing level 55 Death Knights.

There's a lot of content for characters prior to level 55. Like we did for Dustwallow Marsh, we may very well go back and enhance it rather than potentially rendering it obsolete by allowing new level 55 characters for all classes.

Quite a few people (yes, sure, not everyone) like leveling up characters from the very start. And for those that don't, changes have been made over the years to increase the speed of leveling, too.