Of all the playable character exclusions I would be seriously disappointed in, it would most certainly be Shulk. The nearly-unlimited pool of potentiality not only extends its reach to Shulk's properties as a playable character, but also the inclusion of an adrenaline-pumping and emotionally engaging soundtrack, the aesthetically breathtaking stages, and (of most critical importance) a fully-widespread exposure to a worldwide audience. As successful as Xenoblade is (although there some problematic issues here and there), commercial advertisement within Super Smash Brothers can be a greatly beneficial asset towards introducing new players to another series, akin to Fire Emblem.
While there is always the unfortunate possibility Xenoblade's representation would be narrowly defined to only a theme song or an Assist Trophy (as Golden Sun was before), I would passionately argue Xenoblade (and even Golden Sun, for that matter) deserve much better treatment than to be restricted in such a constrained manner. A playable character within Shulk would do magnificent wonders, and thus allow even more universal characteristics to be featured.
Either way, come the next-gen Super Smash Brothers, I am reassuringly confident Shulk (or perhaps another Xeno character) will be fully accessible for game-play, if not already within this generation.
EDIT: I was actually more specifically referring to the Xenoblade Chronicles. Perhaps I should have avoided the use of the word Xeno within the last sentence, as the meaning can be misconstrued to refer to the entire series as a whole, rather than the more recent title).