In an interview with IGN, Overwatch's director Jeff Kaplan said that hero #26 would be on the way, but that they are "not coming out as soon as you think". Kaplan went on to say the hero is very playable but refused to give any real direction as to where the development team was taking the new person. There has been tons of speculation as to who would be coming next, with the most obvious choice on the list being Hammond, the other gorilla who is unaccounted for at the Horizon Lunar Colony.
Whether or not that is correct is pretty irrelevant at the moment as Blizzard has been able to keep a lot of what they have planned for the game under tight guard. Chances are you probably won't hear anything about the new character until Blizzcon comes around at the end of the year. But for now, feel free to hit the Overwatch boards and Reddit to speculate away!