Maybe you could take some villains from the universes the characters are from and have them team up

Also heres the CS for the antagonistic character that wont be making an appearance very soon

Name: Hazama

Gender: Male

Age: Ageless but appears to be in his early to mid 20s

Species: Ghost (inhabits an Azure Grimoire) Human (formerly, with his artificial body made in his likeness)

Appearance: 
[img=http://www.fightersgeneration.com/nx4/char/hazama-chronophantasma.png]

Origin: Blazblue

Personality: Hazama maintained a calm and quiet demeanor around most people, pretending to be harmless as a member of the Intelligence Department. He claimed to hate fighting and preferred to avoid conflict. This, however, was simply a guise to his true nature. When people saw through it, he showed his true colors as selfish and manipulative; destroying other people's lives and twisting their beliefs for his own gain. He had no qualms with killing those who got in his way, and did not seem to show any kind of remorse for his actions.

Short bio: Hazama was a captain of the Novus Orbis Librarium's Intelligence Department and was one of the main antagonists in the series. In reality, he was Yūki Terumi, a former member of the Six Heroes and the creator of the Azure Grimoire and by extension the Black Beast.