Which is a disservice to other villains and wont happen(at least it shouldn't), DC wants to push Black Mask in that role. They can get the same mileage from him as Penguin along with what they probably think is a cooler looking and more intimidating character. They're not going to give a damn about history or 60's tv show. If all that was important then Dr Death, Hugo Strange, Carl Kruger and the Monk would have replaced Joker, Penguin, Riddler/Two-Face and Freeze/Ivy in the first 4 movies and Ra's & Bane would have never made it to the Nolan movies or Harley Quinn and Croc to Suicide Squad. Serious business doesn't care about nostalgia.
I'm not advocating for Slade, I too would rather have him in Teen Titans but if WB wants an inverse Batman as the main villain then Slade is their best bet. Bane has already been used twice while Killer Moth, Catman etc are likely to be laughed upon, Owlman is too contrived and complicated to use.
Similarly if they want a crime boss who is connected to the Waynes' then Black Mask is a more likely candidate.
There is no competition between Penguin and Deathstroke because they're completely different types of villains, it's just that Slade has a better shot at making it if WB goes for his particular type of villainy whereas Penguin has to deal with Black Mask and worse Joker who too has become a night club owning thug in this universe. Penguin has worse to worry about, Slade will make it to a movie anyway, it could be SS, BOP or even JL. Penguin can only make it to a Batman film and even there his chances are looking slim, no pun intended.
Penguin fans should be more worried about Joker, Hush and Black Mask and frankly a good chunk of Batman's rogues. Deathstroke has to compete with an entirely different set of players and in Slade's case his competition is weaker (Bane and Prometheus are the only challenges) and the list is much shorter.