I will argue that the MCU has introduced lesserknown villains and heroes, and the general public has been receptive. I think they'll be receptive to Ocean Master and Ares.
As of today, they may be in the film, but they wont be the main villians. WB just dropped a ton of news regarding Justicr League!
The Geoff Johns era of the DC Universe has begun! Just look at that logo! It really evokes classic Justice League. None of that serious Snyder lens flares in the title.
Speaking of tone, it has been said before, but the film will be "lighter and inclusive." I guess that's fine, but the serious tone never bothered me. It was the storytelling. However, aeeing that this is a team up film, it would make sense.
Official synopsis:
Fueled by his restored faith in humanity and inspired by Superman’s selfless act, Bruce Wayne enlists the help of his newfound ally, Diana Prince, to face an even greater enemy. Together, Batman and Wonder Woman work quickly to find and recruit a team of metahumans to stand against this newly awakened threat. But despite the formation of this unprecedented league of heroes—Batman, Wonder Woman, Aquaman, Cyborg and The Flash—it may already be too late to save the planet from an assault of catastrophic proportions.
Steppenwolf will be the main villain, who was teased in a deleted scene with Lex Luthor in Dawn of Justice. Steppenwolf will also lead the Parademons we saw in Batman's Knightmare sequence.