The Marvel Universe has some pretty silly moments in its history, but we'd like to see these make the leap to the big screen.
CAPTAIN AMERICA LIFTS MJOLNIR
Quick digression: A small thing for fans new to the Marvel Universe's "quirks" from the movies to get used to sooner than later: Captain America is basically Jesus Christ. He always does the right thing, it's always for the right reason and he's always fighting on the right side. Should he ever do the wrong thing or fight on the wrong side, it will almost always turn out to have not been his fault -- and it won't matter because he'll gladly sacrifice life and limb to correct his failings.
He's effectively perfect -- The American Dream embodied in a time-unstuck human to give every other hero an ideal to strive for. Asgardian warriors measure their courage against his WWII exploits. Wolverine defers his leadership. When he died (he got better, because comics) in Civil War, The Punisher made it his personal business to smash up a white-supremacist group for co-opting Cap's image. (The Punisher! A mass-murdering psychopath who respects no law but his own!)
So of course, Captain America can lift Thor's hammer.
It hasn't really been explicitly stated for the movies, but the enchantment on Mjolnir isn't just that Thor needs to stay on the straight and narrow to wield it. Anyone who is "worthy" by the godly standards of Odin can pick it up -- it's just that almost no one but Thor actually is. The list is small: Beta Ray Bill, Thunderstrike and a handful of robots and Asgardian ancients have done it, and it's canon in both universes (certain crossovers are "official") that Superman and Wonder Woman can do it... but Captain America has always been the go-to guy whenever "Mjolnir-lifting" needs to be a big dramatic emotional highlight, especially since the implication is that he could've done it even if he weren't a super-soldier.
I'm fully expecting Cap wielding Mjolnir to be a "moment" in whatever Chris Evans' last Avengers movie turns out to be ("last Avenger standing," hammer in one hand shield in the other, ready to fight to the end against impossible odds...) but what'd be fun would be to introduce his ability to do so as a moment of levity. Like... if The Avengers all take off for a mission and Steve, without even thinking, politely picks up Mjolnir to hand it off to a departing Thor ("Hey pal, don't forget your...") only to suddenly realize everyone including The Asgardian are staring at him, dumbstruck, wondering how the hell he just did that.
STAN LEE IS WATCHING [I know bitch, I was watching]
Let's not beat around the bush: Stan Lee -- by all reliable accounts an overall good guy and the face/voice of the Marvel Universe for generations, is getting up there in years. And while he's had no serious health scares of recent, basic logic dictates that at some point he'll probably have to dial back his workload -- which has included "spotlight" cameo appearances in almost all of the Marvel movies.
Here's a thought (and one I know I'm not the first to suggest): Why not make whatever his "last" cameo turns out to be a doozy?
One of the most powerful (and yet, per the rules of the genre, least effectual) figures in the Marvel Universe is (or, rather was...) Uatu: The Watcher, a nigh-immortal being who exists to observe but not interfere (except in special cases, aka always) in the world's affairs. He can be anywhere, see everything, and has been known to assume disguises.
You see where this is going.
Would it not be a fine sendoff if, after whichever walk-on he's decided will be the last, Marvel's most recent "random guy who looks like Stan Lee" were to duck offscreen to somewhere discreet and reveal himself as having been The Watcher this entire time -- thus implicitly suggesting that even the "other studio" Marvel movies at least have that in common -- before blinking off for parts unknown?
THE DISTINGUISHED COMPETITION
Back when they weren't both IP-farms for massive multinational media conglomerates, Marvel and DC's professional rivalry would occasionally get... well, a little childish. They'd make fun of each others' characters and books, call one another out in editorials, drop dismissive references into dialogue, etc. They both did it all throughout the Silver Age, but Marvel was far and away the champ: They made a whole team of characters (and a universe to put them in) to make fun of DC's big guns.
Originally introduced as "The Squadron Sinister" (later retconned to be a separate, evil clone team) The Squadron Supreme are a team of superheroes from "Earth-712" who are nakedly-obvious analogues of DC's core Justice League: Hyperion (Superman), Power Princess (Wonder Woman), Amphibian (Aquaman), Doctor Spectrum (Green Lantern), Nighthawk (Batman), Skrullian Skymaster (Martian Manhunter) and The Whizzer (The Flash).
The initial "big idea" may have been to give fans a version of the Avengers vs. Justice League battle they were unlikely to ever see otherwise, but in subsequent appearances The Squadron became (alternately) a way to subtly mock DC's seeming "out of touch"-ness by highlighting the goofier or more old-fashioned elements of their heroes versus Marvel's more of-the-moment style and also a way for Marvel's writers to (almost) play with the other sandbox's vintage toys. Still, the order of the day was mostly parody; a typical example being Amphibian's real name being Kingsley Rice. (Aquaman's real name is Arthur Curry. Ha ha.)
The Squadron were perennial "event" enemies/allies for The Avengers in the 70s, but in September of 1985 (yes -- a full year before Alan Moore's thematically-similar Watchmen) then Marvel EiC Mark Grunewald used them as the subject of a 12-issue maxiseries in which The Squadron decide to stop drawing the line at crimefighting and use their superpowers to turn their world into a Utopia -- even if not everyone living there is in favor of the idea. Nighthawk is against it, and when things inevitably go south he ends up leading a revolt against his former teammates. A second reimagining, J. Michael Straczynski's Supreme Power, took the DC-parody angle in an even darker direction.
As far as the movies are concerned? At some point, unless the superhero wave really does come crashing to a halt in the near future, there's going to be a stretch where the Justice League and the Avengers are going to be more-or-less in direct competition at the box-office. So why not acknowledge the elephant in the room and bring in The Squadron to have some fun at The Distinguished Competition's expense? Granted, a whole team of costumed buffoons doing riffs on Henry Cavill and Ben Affleck might be a little on the "light" side for an Avengers event; but maybe they'd be a fun (to say nothing of formidable) obstacle for a more comedy-inflected franchise like Ant-Man, Guardians or Iron Man?
What else do you guys want to see?