1) I would like to see more flak jackets. Other than what you can start with, there's really only two others you can get in the game. And, really, any character can only ever have three total (starting, Hallowed, Frosty). There used to be a Surge one, but you can't find those anymore. By extension, I'd like to see more costumes in general.
2) Re-releasing into the mix stuff that's disappeared. Like the Surge gear mentioned above, you simply can't find it at that AH. There are also tons of accessories that, if I recall correctly, disappear if you sell the item using it. So, if there's no way to reproduce them, it's only a matter of time before this content ceases to exist. And by re-release, I don't mean featuring at the AH for million crowns, that's just ridiculous.
This has been mentioned, what I want is for a new costume/accessory vendor who travels the clockwork with basil. I think this person should be called Silkun as his name contains silk, another trading good. He would sell more/rarer accessories the lower the tire you reach him at.