Looks to me like the first fanfest will be the expansion announcement and a little bit of info, followed by more information in subsequent events with the last event essentially being a prelaunch if all goes well with development and timing.

I expect that it will deal with the empire and won't require Heavensward completion. SE mentioned trying to implement some sort of 'recap' quest or cut scene to fill people in on anything important if they hadn't completed the previous story content. With Heavensward the empire is there, but a lot of it seems to be SE just reminding us that they haven't gone away, so maybe when starting 4.0 without doing 3.0 they'd fill you in on the succession and a couple of other tidbits, but really not much has happened (at this point).