In Iron Man 3, Tony Stark was suffering from PTSD after the battle of New York in The Avengers, and obsessed with finding a way to keep the world safe from another invasion. He built dozens of new suits, integrated his suits into his body, and constantly experienced bad dreams or flashbacks to what happened.
When Happy Hogan gets put in the hospital and The Mandarin (or, really, Aldrich Killian) destroys his Malibu mansion, an unconscious Tony is zoomed off to the last programmed location in this suit's armor, Rose Hill, Tennessee, where he had intended to investigate more of the Project Extremis events. There he met Harley, a quippy kid who helped repair his armor and track down "The Mandarin's" base of operations in Miami.
While it's cool to see Harley show up at Tony Stark's funeral a decade later, the moment also punctuates one of the biggest problems with the MCU's shared universe storytelling, and especially how it's all paid off in Avengers: Endgame. It's long been said that the shared universe model hurts individual films since they're all ultimately setting up something bigger down the road, and this is especially true for the MCU and its famous mid and post-credits scenes. The movies are all well received and perform well financially, but the excitement is always forward focused, with each movie delivering a reason to be excited for the next, sacrificing complete movies for sequels and bigger shared universe teases.
It's hard to argue with the model, as the MCU's success has shown, but it does hinder the potential quality of a single movie for the potential quality of an entire universe. Then, when Avengers: Endgame comes around, billed as the three-hour culmination of the decade of movies that came before, there's, unfortunately, simply not enough time to deliver on all these promises at once.
Avengers: Endgame gave both Steve Rogers and Tony Stark get excellent conclusions to their character arcs and are sent-off in a respectable fashion, but when it comes to providing impact on the rest of the MCU's set-up, most of the emotional punchlines don't have more breathing room than a moment that could be misconstrued as a throwaway Easter egg. Harley's appearance at the funeral, for example, implies Tony had kept up their relationship, and may even imply Harley could become Iron Lad in a future Young Avengers, but for most audiences it was nothing more than a blink-and-you'll-miss-it moment.
There's simply too much in the first 22 movies of the MCU for the Avengers: Endgame to assume the burden of giving every single moment a satisfying pay-off, but that means some seeds that were planted in earlier movies don't see more payoff than a wink and a nod, ultimately making the MCU is less than the sum of its parts, delaying its payoff to the end of the road where, in reality, it can't stack up to the setup, like the Harley cameo. This is also illustrated in Avengers: Endgame's final battle, where dozens of characters are all brought to the big screen together, but the movie is really only able to focus on a handful, with most of the returning snapped characters getting little more than a cameo, with Spider-Man and Doctor Strange being the only two characters to get anything resembling a proper an emotion beat.
Given the movie's inability to give a satisfying conclusion to every single established MCU thread outside of Easter eggs like Harley, it makes sense that Kevin Feige has indicated the MCU might evolve the formula now that Phase 3 is done. Whether that means the larger arcs are smaller, the crossovers more abundant, or the whole notion of multiple phases making a saga going away, it seems the MCU will change in ways that hopefully address this problem.