i have two theories about finale-related things, one for each of the new characters
with shining armor, I get the feeling the others are going to want to know about Twilight's brother and why she never got around to speaking about him. she's probably going to give some sort of explanation. perhaps she doesn't like him that much? perhaps he was gone because of political/war reasons, leaving her alone? I want a background story. yes please.
with princess cadence, I WANT her to be the next evil antagonist. I get the feeling that's not going to happen though, Hasbro has been (or plans to) selling her as a wedding princess, I don't think making her evil would work out well with profits. what's probably going to happen is some new antagonist is going to crash the wedding, capture cadence and/or shining armor, thus adventure to save one or both.
also:

THAT WAS FAST.