Almost all players know that there are Overwatch characters in Heroes of the Storm, and it's not like it's a bad idea. Blizzard, being the studio that it is, simply knows how to connect the dots and make the community happy. Now, there are speculations as to which hero will be next. Actually, this has been answered already -- it is no other than Genji.
In a Reddit post from user rost473, he shared a leaked image of a Russian steamer on YouTube attending the Heroes of the Storm 2.0 Summit in Paris. He's convinced that the cyborg ninja in Overwatch will be the next character, as it's somehow visible in the aforesaid video (embedded below). This was shot at Blizzard's Versailles, France office.
The image is somehow imperceptible, making it a bit hard for a person to see or distinguish. However, if one is to zoom out, a character selection screen can be seen. On it is a portrait that looks exactly like Genji of Overwatch. Of course, if this is true -- if the character becomes part of the title -- he will be on a different look (but not necessarily changed).
According to Polygon, the aforementioned portrait is not something that Overwatch players should be satisfied right away. Interestingly, there are other proofs or clues that point to the possible arrival of Genji in Heroes of the Storm. Just recently, players received a very interesting email after signing up for the version 2.0 beta. Although it was removed already, the character's page source clearly states "the deadly assassin Genji" as among the characters of HoTS. There is also a possibility of an OW-themed battleground coming to the game soon.
It remains a mystery if whether or not Genji from Overwatch will arrive to Heroes of the Storm. However, if he does, Blizzard will definitely make some teasers or official announcement. It's only a matter of time before things will finally be unveiled.