10) The Empires
A DLC solely based on large-kingdom and empire troubles. Specifically, succession crises, break-away factionalism, a reworked Crown Authority system that can better model the HRE (because the difference between Limited and Medium CA is a massive jump that probably could almost never have happened in the actual HRE). This DLC will also include in the patch a mechanism for titles to disappear if the title holder holds less than 50% of the de jure land area for 2 generations. IE: If the "Holy Roman Empire" consists of Frisia and nothing else, nobody would recognize that title as having any meaning. So the holder is HRE, the heir is HRE with an icon letting the player know the title is in danger of being dissolved, the heir (or heirs to a few generations) to that gets a special type of claim allowing the restoration of the title at a steep discount with a huge prestige payoff, and then eventually the claim dies out.
11) Fortuna, Myths, and Legends
A DLC that adds several hundred flavor events, many of which have minor effects, maybe a few traits added. For those having WoL, the patch includes a much better focus balance and more interesting things to do that aren't game breaking. Use teaser Dev Diaries to showcase a few event chains to get everyone all frothing for it.
As above posters have said, the "historical" myths. Having someone questing for the Holy Grail, having villagers claim to have been attacked by regionally-appropriate mythical creatures because let's be honest, we all want a Werewolf in London event and we all think it would be awesome for the Boyers in Wallachia to revolt and burn stuff down thinking their Count Vlad the Impaler (Son of Vlad the Dragon) is a vampire. The DLC doesn't have to actually *include* werewolves, vampires, witches, etc. But the EVENTS would be awesome. "My King, a peasant woman claims her child has been carried off by the Fae. A) Send the Marshal to kill the Fae! B) Send the Steward to investigate C) Let the Spymaster scheme a way to use this to my advantage! D) (requires Lunatic/Possessed) We must find these Fae and invite them to court! E) (requires Cynical) Toss her in the dungeon"