I like the idea of an invincible hero that has the downside of not being able to fight back. He’s all taking it and none of dishing it out. You can’t tell a typical cape story with that because the audience will never get their big fights, so you have to get creative. You do now have a character that sits between heroes and villains AND he can just go straight Looney Tunes in conflicts. Villain has a giant doom gun pointed at the city? He can do the ol’ “stuck your finger in the barrel and let the gun blow up” bit. Have a menace with enormous power that lives in his own place with diplomatic immunity while he plots takeovers? Our hero can be dropped out of a plane onto his castle and just... hang out until the villain caves and agrees to concessions. The villain can’t kill the guy, teleport him away, or magic him into submission, so our negotiator gets to sit around the castle, eat its food, be annoying in meetings, throw sinister blueprints out with the trash, and so on until everybody agrees to come to the table. And because our hero is neutral, everybody still kind of has to respect him whether they hate him or not. Top secret extra judicial “hero” spy team is violating everybody’s privacy? Well then our negotiator pays them a visit and can wander around their fancy fortress ships reading up on all their secrets and the spies can’t kill him any more than the villains can. I like a character that can force comic book characters to have more interesting conflicts and grow the world in ways that don’t revolve around violence, but that is still fun himself.
So if we have one invincible hero, what kind of team can he make with people possessing similar phenomenal power kept in check by balanced downsides. It was fun thinking of a team of heroes made up of just characters with immortality powers:
Shrug as the captain is invulnerable but can't do damage back.
A character with Willow-like rebirths. Once the body dies, a new one crawls out of the ground. So they’re able to do damage but they aren’t super strong or anything. You can knock them down as much as you want but a few seconds later they’re just going to be back.
A regeneration character. Similar to some other famous characters out there but without the bonuses of claws or swords or buffed up super science skeletons. With near instantaneous perfect regeneration, you can’t knock them down so they’re just going to always be on you, poking and prodding and annoying you and they won’t stop.
A character that's a robot. She can actually be built with weaponry and do damage but she’s balanced out by being the only one on the team that’s fully physical and can be temporarily “destroyed”. Every time she "dies", they rebuild her body and upload her consciousness back into it, so you can’t ever completely stop her. She just needs a lot of spare parts on hand.
A character that has the intangibility of a ghost but none of the other usual “powers”. They can’t levitate or fly or possess things and they’re not invisible. They’re just around, you can’t physically interact with them but they also can’t physically interact with you either. On the one hand, they’d be a great spy since they can just walk through walls to get your secrets but on the other hand, you can still see them. They’re the perfect “annoyance” character since you can’t get rid of them and they can just hang out, sing off key, laugh at you in the shower, and prevent you from sleeping until you cave and do what they want.