Given that Tracer is more or less the frontwoman for Overwatch's big gang of fighters, it's not too shocking to discover that she'll be the first Overwatch hero added to Heroes of the Storm.
Overwatch will be launching this month, so I suspect that Tracer might be made available as a hero by buying the game on your Battle.net account, as well as being available through the typical HotS means.
In addition to the rumors that she'll be there (which seem likely, as there's a pretty believable screenshot,) there are other rumors that Tracer will be able to auto-attack on the move, and that she will get to use her heroic ability from level 1 - but that she'll get to refine it in different ways when the team hits level 10.
While Overwatch is the first Blizzard IP not to really have a "story mode" single player game, they're doing tons of concept and art work to flesh out Overwatch's rather huge roster. I think we can confidently expect these shooter characters to make their way into Blizzard's MOBA.
Also, given that both Starcraft and Overwatch have more of a sci-fi feel than the fantasy feel of Diablo and Warcraft, it'd be nice to see some sci-fi themed maps come to Heroes of the Storm.