Remember when select people based an entire leak off of whether or not the "same tree" was found in a reflection? This is kind of like that, but in-game!
Taking to Reddit, f1bermau5 shared a discovery that seems to imply that Heroes of the Storm will host yet another Overwatch character very soon. Blizzard insists that multiple characters are still coming to Heroes this year, and a recent tease (which covers various battlegrounds in snow) implies that Mei is headed to the Nexus.
This is further backed up by the "same snowflake" theory, which compares and contrasts the snowflakes from Overwatch proper to the ones in the Heroes event; finding multiple matches. Now this could be a case of Blizzard re-using assets, but as a general rule, adding Overwatch characters to Heroes results in a cross-game surge of new players. It would behoove Blizzard to keep that gravy train rolling, so Mei seems like a no-brainer, alongside of a potential Overwatch crossover event and the long-awaited D.Va rework.
Snowflake comparison [Reddit]