Sinister Six

It’s time for the villains to unite against one or many heroes.  Such a faction of evil can only be stopped by . . . Well, that’s a long list ranging from punches to infighting to whatever you can imagine.  Maybe they never figured out how to get out of their super secret headquarters, so they only got to threaten people.  Either way, your bad guys are working together and that means you’ll be tempted to something that nobody (except for everybody) has dreamed of doing.  That brings us to:

1. Villains do NOT have to betray each other.  I’m getting my big pet peeve out of the way.  Yes, they are bad guys and loyalty isn’t their thing.  We make them more evil by having them betray others or off a minion for sneezing without a tissue.  This becomes cartoonish though.  People are so used to this ending that they aren’t surprised by the back stab.  Even a double, triple, or quintuple betrayal isn’t as shocking as it was years ago.  Never think that this is mandatory.
2. If your villains are all from the same power tier then you should have them act as equals.  There could be one who put the group together, but turning previously high ranking baddies into minions will hurt them.  Who will believe that they are capable of being a leader in the future when they were flunkies this time?  If you do this then you need to give a good reason as to why they bow to the leader.
3. When they interact with the heroes, you will find yourself wanting to do the traditional foe switch.  The villains swap enemies in the first encounter in order to get the upper hand.  It’s a logical trick since they are ready for a new adversary, but the heroes are being caught by surprise.  I won’t say that this is a cliche or shouldn’t be done, but you need to be careful here.  Try to do it with a twist or accept that it’s unavoidable.  Best not to draw it out too because this is where the heroes may not be united too.
4. Villains are secretive, so don’t have them share everything about themselves.  Special abilities, lairs, and histories are among the things that they won’t blurt out.  This connects to #1 because they could be expecting a betrayal or having some reason to maintain a secret.  It could be something as basic as the villains worrying that the other can be captured and reveal everything.  This can be a secret that damages the team when it comes out, but it shouldn’t be that bad if it helps things.
5. Goals are going to be a little difficult here.  You can take the path of least resistance and make it entirely about defeating their associated heroes.  The villains can see each other as a way of winning their feud.  Beyond this idea, you need to create common ground for them, especially if they aren’t similar in motivation.  A thief won’t work 100% well with a murderous psychopath since one wants sparklies and the other wants to kill.  Maybe they have a target that satisfies both interests or a third party to unite them.
6. They don’t have to name their team.  Seriously, stop calling everything a League or Brotherhood.
7. A team up is an interaction between people who may very well have strong personalities.  This means, they have a high chance of influencing each other to some extent.  It may be that one learns some new tactics to use when they’re solo again or they develop a trait that changes how they operate.  While ego means villains are hard to change, it doesn’t mean that they are devoid of evolving in some fashion.