You can read more about the new game from the Informer Article. It sounds like what we saw on the trailer are more 'cutscenes' than actual game itself, which will be more puzzle oriented.
You can find scans here:
http://kingsquest.wikia.com/wiki/KQ9_developmentInteresting details is it confirms the Gwendolyn as the daughter of Alexander, and Gart will be the son of Rosella.
The previous games are all canon to this game. It is not a reboot, but its also not a sequel, but a reimagining. Or rather perhaps better to say the framing story is a sequel, but each episode will each tell a separate story in the life of King Graham. It will avoid retreading any previous King's Quest game, with the exception that the first episode begins with a prologue retelling Graham's encounter with the DRagon in KQ1. But it won't be King's Quest 1 HD. THe rest of the game will flashback and have you experiencing how Graham became a knight of Daventry. Some later episodes will chronicle Graham's young romance with Valanice after returning from Kolyma. There might be an episode that relates Graham's life as a seasoned knight between when he was first accepted into the ranks, and when he became king.
Who knows what else later episodes have in store. There are five episodes planned in total. Choices made at various points will have consequences for later events in later episodes. They also influence the choices that Gwendolyn makes in her 'present' period storylines. Choices are also tied into the scoring system which divides choices into Wisdom, Bravery and Compassion. Which means many points in the game require one of the three choices to be made depending on those factors. Choices are not done through conversation trees, but through physical actions that Graham takes in the environment. So they are themselves usually item puzzles.