1. Brill and Sunstream look identical as they were shown in Discovery - so this option would be the most likely. Would end the speculations, whether there will be twins born to the twin-couple.
2. In the same poster Shuna is shown as a very old woman. In case the whole poster is an exact "snapshot" from the storyline, this cubling must be a much younger sibling of Sunstream's and Brill's firstborn child(ren).
3. Not much evidence for it - just fan speculation due to the fact, that Ember and Teir do not show in the poster. And that they will most probably recognize in the Final Quest. And some other teasers hint that something bad might happen to Ember.
Last not least ...
4. This teaser is very possible a collage of scenes, not a snapshot. And as far as I've got it not really new ... so things might have changed in time passing between this picture and the actual writing/drawing up of the Final Quest.
And YES ... this is ALL about fan speculation
Easter EggQuest 2012 found