User blog comment:Jimeee/Hearthfire confirmed as the latest add-on. Official trailer./@comment-90.215.96.212-20120828211318/@comment-1923896-20120828221942

Who says they mightn't appear in a later dlc?

It seems to me like Bethesda are kinda setting these dlcs in stages, so Dawnguard armour is weaker than ebony armour, so Dawnguard should be done around midgame. With Hearthguard, you've probably been adventuring for a while and decide to set roots, also around midgame. Riding dragons? Apocalyptic scenario, so probably endgame or as a reward for stopping Alduin (Ok, not the best example but given that the first two dlc have been vampires and building a house, theres a lot of room for interpretation)