We've talked quite a bit about Sylvanas and her history with Arthas in the past, but let's not forget that there is another woman Arthas has a nice long history with on the opposite faction: Jaina Proudmoore. Jaina's biggest moment in WoW is probably the Missing Diplomat quest-- after a long goose chase to find out where the lost King of Stormwind is, all you do is end up finding the man who betrayed him, and then Jaina thanks you and wishes you farewell. Wha? Players have always though the quest ended a little too quick, and Blizzard has admitted that they plan to continue the storyline at a later day.
As in, during the next expansion. When a player asks what happened to Jaina and the Missing Diplomat, Drysc mysteriously says "Northrend holds many secrets, and many answers." So it's a good guess that we'll find out at least another piece of the puzzle to where the Lost King of Stormwind has gone.
And it's also a pretty good guess that Jaina will be there, somehow. Despite the fact that she's pretty much the strongest leader the Alliance has, she's also pretty buddy-buddy with Thrall, and she's got that history with Arthas-- they were friends (and possibly romantically linked) before he went all power-crazy and possessed. There's no question that some of the answers we uncover in Northrend, whatever they may be, will involve Jaina Proudmoore.