http://imgur.com/M6oZk8J
If you look this screenshot from the "We are Guardians" trailer it appears to be a Fallen Ketch and some Awoken ships.
[b][u]"Wanna have Vex"[/u][/b] is a stronghold of knowledge about The Fallen and he will be helping moderate responses on this post.
[b]Theory 1[/b]: Could we be fighting alongside the Awoken and The House of Judgement (The house Variks belongs to)
[b]Theory 2[/b]: Multiple Fallen houses are working together with us to destroy Oryx.
[b]Theory 3[/b]: The Ketch's seen are just awoken using Fallen Ketch's
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Epiphany time, I think variks will unite the houses during this threat and become the Kell of the House of Judgment. The fallen are after the traveler we all know that by now, maybe at some point in the story of TTK variks and the queen will make an alliance with the fallen houses, and because the city won't have an option, we will have to fight along side them. Who knows maybe we will have our own squad of fallen soldiers to help us overcome our mutual enemy. The enemy of my enemy is my friend. Here's to hoping!