We've seen Batman villains -but also just villains in general- defy each other, fight and do horrible things in order to prove their superiority to the other criminals. And that's very in character for them, since most of them are pathological narcissists and each one of them has their own goals and aims. For instance in Gothtopia, or in the recent War Of Jokes & Riddles, two of the best ultimate Batman villains -the Joker and the Riddler- fight each other about who would kill the Batman first. And it wasn't just a race to who finally ends vigilantism, but just their best way to prove who's the best villain.
But in the last few episodes of the tv series Gotham, The Legion of Horribles is gathering. They're a team made up of Mad Hatter, Scarecrow, Mr. Freeze, and Firefly who are apparently devoted to Jerome’s terrible plans. This group is particularly terrifying, and the fans fell in love with the idea very quickly.
Not that villains have never teamed up before, there are a few stories (or AUs ,like The Enemy Within) where they cooperate. But it's been a while since that has happened in comics, and a long-lasting team up might be a good way to create new, hard challenges for Batman.
The villains that would make an interesting team are the ones who wouldn't like to be in a group in the first place, but who recognize that cooperating might give them many advantages.
Scarecrow, Poison Ivy, the Riddler, Two Face, Mr. Freeze, Mad Hatter and, to top it off, Joker.
What if that happened in comics again? This time no one is the leader and they all have a common goal, which isn't only killing the Batman, but perhaps them trying to get something else. The terror, the chaos, perhaps the betrayals could make a very good story just like the battles between them do.
Each one of them has their abilities, resources and a high IQ, plus a relative connection to the Batman himself.
Fear toxin, “killer” plants, laughing gas, freezing gun, hypnotism and complex schemes; what better team than this one, with such fascinating characters?
Gotham seems to understand how much fans love seeing villains in all of their evilness, although they didn't have too much screen time, and how making them collaborate might make the story even more compelling by challenging the good guys in ways we've never seen before!