This one's a bit of a long shot, but it's also one that seems to be a pretty popular theory around the web. Sure, it's possible Mallory could be the next Supreme: she's a very powerful witch whose bloodline, according to Zoe, stretches all the way back to the original witches of Salem. The thing is, Cordelia has only just ascended to her role as Supreme. From what we've learned so far, Supremes tend to hold their power for decades, ruling over a new generation of witches.
Then again, Fiona held on to her position as Supreme for far too long, past her expiration date. Is it possible Fiona's selfishness ate away into Cordelia's time as Supreme? I mean, it's not impossible. I personally think Mallory is not the next Supreme, though.