I at least expect 2 trios; one under xerneas command and the other under Yveltal. I think there should be more ground types and poison type legends, those are the least used for legends, so it would be nice to have at least one legend with any of those two, maybe alone or in combination with another type.
I never liked how there was more than one side trio, such as Generation V having both the musketeers and the kamis. However, this idea for having 2 side trios, both tied in with Xerneas and Yveltal, is very palatable for me. :)
As for what I expect of a trio, I anticipate that they'll imitate the previous generation, with again, having 2 trios (even though technically one was a quatro, that's not the point.) Although I don't like it, Gamefreak has been, for the most part, squeezing in more and more legendaries per generation.
I feel as though some people are overemphasizing what they're expecting of unique types for a trio. I'm more interested in the actual concept behind them; for instance, despite Gen IV having an mono-psychic trio, it explored three parts of the human psyche--wisdom, emotion, and willpower--along with how their origins tied in with the over-arching theme of Sinnoh's creation story. There could have been more uniqueness to them, yes, but having a bunch of "unique" types that didn't tie into an over-arching theme would have annoyed me.
Not much has been revealed about what Xerneas and Yveltal's roles are. There is overwhelming evidence, however, that they are based on
Norse Mythology. I don't have much knowledge about Norse Mythology, but I want to see a trio that's based on other Norse gods! {<3}