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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There's not one, but FIVE Ketches. Looks like we may be fighting alongside ALL the Fallen Houses.