A few months have passed since Sombra made her appearance in Overwatch, so it should come as no surprise to hear that Blizzard has begun teasing the 24th hero. Unlike Sombra, however, it appears they are being a bit more direct with the reveal this time around. We still don't know who it is, but as you'll find in "an interview with inventor Efi Oladele" Blizzard is strongly hinting at some sort Omnic hero.
And before you ask, no, its not Doomfist. Chances are high Doomfist is in development given how many rumors and leaks appeared over the past few weeks, but it has been pretty much confirmed that the 24th hero is not someone we've seen mentioned previously. Similarly, there have also been rumors that the 24th hero is some sort of Omnic spider-tank as seen in the in the DICE 2017 Summit, but that has also been debunked. So until Blizzard themselves reveals the hero I would recommend treating these types of rumors as just that.
As for the release date, it might actually be next week. It took Ana one week to go from "brief teaser" to "full reveal", so its not unlikely for them to follow the same method for the 24th hero as well, especially since Sombra's 'eternal' ARG ended up being so disliked. Whatever the case may be, there's going to be something interesting happening on February 28th once Season 4 of Competitive Play begins.