The Rules have been published and Lurker doesn't contain that many new Investigator cards. Aside from the relationships, there are 12 spell cards, four Common Items and four unique Items. So, given we have no idea what's inside and this is all idle speculation - let's guess, based on utterly unfounded speculation, as to what do you reckon those cards are?

Another Elder Sign (bringing the grand total to six - enough to win a game) could be one - the new gates would be enough to justify this. Another weapon of some sort - perhaps some kind of Thuggee Kukri that required a sacrifice of one stamina (blood) to be used.

Spells: Keenness of Two Alike: a -3 check. Discard to avoid being Devoured. Some kind of attack spell - Maybe one that was easy to cast but required you discard a Clue token to get a +4 bonus.

Common Items - A Blunderbuss - 2H, Physical. +4, discard an item to gain an additional +2 bonus. A Diary - you don't discard clues when you are injured or go insane. the Diary does not count towards Items when you are driven insane.

There's no way they're adding another Elder Sign. A lot of players find relief that the addition of expansion cards dilutes the Elder Signs. The only reason Kingsport came with an Elder Sign was that Wendy started with one.