There are two issues I have with the separate universes approach (although I admit JLA/Avengers did it well).

Firstly, you have to keep coming up with reasons for dimension-hopping. Some big bad has to threaten the multiverse. We have to spend time setting up the trip between universes. And one explanation is gonna pretty much be like another as time progresses. Simply having Clark Kent show up on assignment in New York City, or Bruce Banner seeking out S.T.A.R. Labs, is a) simplistic, and b) doesn't require a lot of set up. In less than a page, a modern crossover could have Bruce Banner simply hitching a ride to S.T.A.R. Labs or show Hal Jordan's aircraft entering Latverian airspace. 

Secondly, for me, it sucks the fun out of things and is a bit too meta-fiction for my liking. Yes, the heroes commenting on each other's Earths in JLA/Avengers was fun, but there is always an exception to the rule. I like to suspend disbelief. Superman telling someone that he has heard whispers about Galactus' appearance from heroes in another universe, or the slightly fanboy aspect of DC heroes commenting on the hostility of Marvel Earth civilians, is something that indirectly reminds me I am reading fiction - rather than allowing me to fully engross myself in the tales. I know I'm reading fiction, but I don't want to be reminded of it!

Simply having Clark Kent show up in New York keeps me fully absorbed in the tale I am reading. But that's just me.

For me, canon is important because it's about taking ownership of a fictional universe, being accountable, and being creative.

Weren't DC's Star Trek: The Next Generation (and TOS) comics canon? I thought I'd read they were. And for me, that means the writers had to be creative. The writers on DC Comics' TNG comic came up with some tremendous storylines perhaps *because* of it being canon. They couldn't just have a planet or ship blown up. Or have Q killed. Or feature Starfleet infiltrated. No, because they had to write within the constraints of canon, it made it all the more worthwhile and satisfying. It really did.

Scenario 1: a non-canonical TNG comic could have an editor say, "Oh, just blow up the Klingon homeworld, it's not like we have to fit it in with the TV series."

Scenario 2: a canonical TNG comic would have an editor say, "I want a major threat to the Klingon homeworld, but think about the constraints from Paramount. We've got to make this threat work, but it has to be done this way and that way - after all, the Klingon homeworld will be featured heavily in the next TV season."

Without canon, and this is just a personal view, there aren't consequences. If a non-canonical TNG comic features a particular homeworld blown up, or a character's family member hurt, what does it matter? If it's non-canonical, it's as disposable as yesterday's toilet roll. However, if it is canon, it adds something to the mythos. It becomes part of a fictional universe's tapestry. That is why it matters to me.

Superman and Spider-Man is my favourite DC/Marvel crossover. I wish it was canon. That it can be explained away as happening on one of a trillion Earths, if it really happened at all, irks me a tad. 

But how do we know they don't? We only see what we see. Perhaps suspension of disbelief can lead to us imagining that there's an "Untold Batman/Iron Fist encounter" that occurred. We haven't seen it yet. But we will if DC and Marvel ever do it. 

Iron Fist and Cap don't run into each other all of the time. Spider-Man and Green Goblin can tear up New York time and time again without Cap, Iron Man, Thor and the FF intervening. Where were other heroes when the Matador was cleaning Daredevil's clock? Juggernaut often causes a rampage in New York, but you never see the FF or Iron Man showing up to deal with him, usually the X-Men or others.

Everyone's view is valid, but I am of the mindset that what you have never seen doesn't equate to it didn't happen. We haven't seen Iron Fist take a bath but I'm pretty sure it has happened. Suspension of disbelief and all that could lead us to say, for instance, "Superman and Captain America have had numerous phone calls and encounters, we just haven't seen them published yet." Perhaps the Avengers and JLA, or some representatives, meet often for intelligence-sharing purposes, we just don't see it because we don't see every aspect of a character's life.

And that doesn't just apply to intercompany crossovers. I might have bought 12 Spidey comics back in the day, but I didn't presume Spidey had 12 battles a year. I presumed he had others - we just hadn't seen them yet. And maybe the "Unseen Spidey stories" included him teaming with Batman, Flash, Aquaman, etc.

Cinema and comic books are different media, and demanding that they synchronize continuity sounds like a way to straitjacket the writers. For instance, Chewbacca was killed in a novel during the long years (both real time and story time) between Return of the Jedi and The Force Awakens. But it would be silly for the filmmakers to feel bound by all the continuity of the Star Wars Expanded Universe. So they didn't have to say "Chewie is alive again because X." Instead they rightly ignored the book.

Similarly the MCU has seen great success by treating the comic books as a smorgasbord of possibilities rather than a Bible that may not be transgressed.

Even in comic books, dumb precedents get ignored all the time. Remember when this happened (Action Comics #267, Jerry Siegel)?



That was super-dumb to think that if the Legionnaires met Superboy, then it would have to be a different generation of Legionnaires who met Supergirl. A different generation who had exactly the same powers, code names, and appearances as their parents. Dumb. So it was just forgotten. (Until Grant Morrison resurrects it for a gag in the near future. You heard it here first.)

Canon is not what makes stories meaningful. Good writing is what makes them meaningful. Past stories might (and often do) provide good material for future stories, or maybe the opposite. When Joss Whedon took over X-Men, he wanted to tell a "Kitty and Colossus" story to pick up abandoned plot threads from the mid 1980s. But Colossus had been dead for years. Whedon didn't completely ignore the canonicity of Colossus' death, but he didn't make a six issue "Resurrection of Colossus" arc out of it either. He just had the characters note the inconsistency in four panels, and he moved on. In his work on "Buffy the Vampire Slayer," he called this "hanging a lampshade" on continuity chasms.



DC understands the need to reject continuity too, which is why they keep rebooting the multiverse. But they also realize that fans are overly attached to continuity, so they feel compelled to have "event stories" to justify the reboots.

Honestly the only Marvel/DC crossover I've ever liked was Batman/Captain America. Part of it is that anything set in the past is going to get a leg up on me. Probably more importantly is that it was pretty much treated as an Elseworlds book which made the issues of how the x-over came about moot. And it was early enough in the career of Captain America that it didn't matter that they'd not run into each other before.

The various Planetary cross-overs (though I like them a LOT less than any other part of the book) are easier to deal with simply because of the nature of the book.

But overall...Bleh!

Funny you should mention that ... because DC's rebooted the Legion a bunch of times but never caught the balance between pleasing both old fans and new fans ... I've thought for some time this is how they ought to handle the series. Go back to the original continuity but gives us the next generation. Some of them would be almost indistinguishable from their parents, some with two Legionnaire parents might have powers combined in interesting ways, and some would be completely original.

Getting back on topic ... honestly I don't have a preference, although as others have said a lot depends on the type of story. I never got all that enthralled with JLA/Avengers honestly, but it did have its high points. But most of these crossovers are relatively small scale. Most Batman stories don't benefit from becoming interdimensional epics. So for the most part I prefer 'Earth Crossover' as a venue unless a story truly merits it.