Andy E. Nystrom wrote:

One interesting variant seem in superhero comics is the Costume Party Sequence. While characters with dialogue generally dress as indigenous characters, background characters sometimes dress as other characters. This neither confirms nor denies the character is indigenous but does confirm that the character wearing the costume is aware of them. This may be a form of cameo but differs in a couple ways: the character may appear off and on throughout the sequence, and it's generally confirmed by partygoers and readers that the character they're dressed at isn't actually present. Though this may be an incorrect assumption, as sometimes characters show up at a costume party dressed as their actual alter egos.

Yeah, the costume party one doesn't prove an actual version of the character exists in a given universe, but does mean that there is at least awareness of the character, even if only as a fiction. And just as in the real world, you can have a mix of people dressed as fictional characters alongside real world individuals. 

I can think of two occasions where comics depicted people in costume, and while some were clearly just regular joes dressed as the characters, others had to be the real deal.

Charlton's Ghost Manor II#21 included a bunch of cameos at a parade, featuring Blue Beetle, E-Man and Captain Atom, and while the location might suggest they could have just been people dressed up as those heroes, they each demonstrated evidence of their actual superpowers beyond what a normal person in costume could achieve - E-Man appears to be firing energy from one hand, Captain Atom is launching into flight, and Blue Beetle has access to his flying beetle ship.

Meanwhile 2000AD Prog#2083's cover features a group of characters attending a convention, surrounded by people in costume. Some are definitely just fans dressed in cosplay, such as a female version of the fourth Doctor leading round a man dressed as a humanoid version of K-9. This might suggest the same to be true of most of the other characters present, until you spot Mike from Monsters Inc. and realize that there's no way a human, even a small one, could wear a costume that looks like the real Mike, with his extremely thin legs and arms.

One thing that often comes up when discussing these kind of crossovers is the "validity" of same. Those who dislike the concept that the Omniverse means every story is linked often use this to argue that various crossovers don't "count." As such, there's a general "loss of credibility" in a crossover if the story is underlyingly humorous or seems to be self-aware or metafictional, even if all other aspects of the crossover would otherwise support it being canonical. 

An example of what I mean. The more curmudgeonly among us thus likes to insist that it clearly wasn't the "real" Judge Dredd who chatted with the "real" Marshal Law in 2000 A.D.'s "A Night to Remember" because that tale was a celebration of the comic's history and somewhat tongue-in-cheek. While I disagree, I do recognise that it's better to provide non-humorous links when they are available, as those are harder to refute. Naysayers can insist ANtR doesn't prove Dredd shares a multiverse with other 2000 A.D. characters, but it's much harder to deny it when the crossovers happened in Helter-Skelter, a serious story which was all about scientists in Mega-City One accessing alternate realities. 

Andy E. Nystrom wrote:

One interesting question of whether or not something counts is bonus features on discs or tapes. Short films certainly count. I would argue that Spider-Man "meeting" the movie X-Men probably doesn't count despite appearing on an Easter Egg in the X-Men DVD because the actors immediately broke character upon seeing him. But what if a character (and I don't have any example in mind) appears in a deleted scene that appears as a bonus feature in a movie or show but did not make it into any cut of said movie or show?

I think if everyone remains in character, and the cut scene doesn't contradict the finished version (e.g. it could still have happened, just out of the audience's sight), then it counts. And if it does contradict the finished version, then it still counts too, but as evidence that the given character is in an alternate reality where events played out slightly differently.

Yeah. Fan made stories and art would also fit, and thanks to the internet have a level of staying power. Given that these can create crossovers without any restrictions and are so numerous that it's virtually impossible to maintain track of them, trying to list them all would be insanity inducing. Plus while there's some incredible fan art out there, there's also a ton of terrible stuff. If something makes it to proper publication, then I'm cool with adding it to the list, and I am willing to make an exception to the "no tracking fan art" on occasion.

Foreign swipes often fit neatly into the other categories - I listed examples of Marvel and DC characters appearing in 2000AD humour strips like Captain Klep in the unofficial cameos section, and given 2000AD is British, they'd count as foreign swipes. 

Andy E. Nystrom wrote:

I thought of one more type, or at least an interesting variant of temporary world merger: fictional characters in the real world. Sasha Baron Cohen's characters have interacted with many real people (often oblivious to the fact that they were talking to fictional characters) while Spinal Tap has performed in character in the real world. And of course Space Ghost has interviewed the real versions of many people for Space Ghost Coast to Coast. So although Ali G et al, Space Ghost, and Spinal Tap have never met as far as I know, it's possible to link them via the real world.

I think the only problem with that is that it would effectively link any stories set in "the real world" as most of them feature real world people at some point. People like the US Presidents could link a ton of comic company universes, for example. 

It's a bit like: I'm willing at a certain level to accept using Predators to link realities, but not real world species like dogs, cats or humans.