Wild guess with very little basis in reality that's been baking in my mind overnight: Dark Nexus II with Tassadar rework + skin.
Why? Tassadar's rework has been in the works for more than a year now. Yes, literally. And they've been dropping sly hints over the last few months that he'll be done "Soon(TM)". And with the reduced cadence, there's no way they're dropping the rework right next to a hero release. They'll want to spread out the hype, understandably.
And why would a new hero be teased with a picture of the Raven Lord? Specifically one where he's holding a Singularity. The last time we saw this we got the Dark Nexus. It would be a great opportunity to do a "sith"/Tal'darim Tassadar skin (for those not in the loop, the Tal'darim are Alarak's tribe of protoss).
All of the above combined with the fact that they exhausted massive amounts of resources on Deathwing truly makes me think that this is too soon for a new hero. They haven't said it outright, but reading between the lines in comments made since Deathwing's reveal would suggest that they had to crunch to get him out before the end of 2019. Efforts like that have a cost, and I unfortunately believe we have to pace our expectations for this year.