I wouldn't even go that far. Forget having the starring superheroes appear. I'd settle for them just being allowed to say their name. (I'm sure scheduling issues are the probelm there, too, eh?) I'd love to see cameos of relevant minor characters:
DD1: Since this is the one that sort of built Marvel's HK from the ground up, perhaps no cameos here. Still, a shot of Avengers Tower in the background wouldn't have gone amiss.
JJ: The Waitress from Avengers serving JJ and LC at the cafe. Use of the characters' actual names as opposed to 'The Flag Waver' and 'The Big Green One.' Ideally there'd have been a whole conspiracy with one of the Avengers leading to a cameo with them, could be anyone. Even Rhodey.
DD2: Black Widow, evaluating him as 'not Avengers material' to protect him. If not her Maria Hill. Helen Cho being involved in the hospital fiasco. Falcon testifying that Frank didn't have PTSD based on their interaction at the VA.
Luke Cage: A character played by Michael K. Williams, an incredible actor (most famous for The Wire) who cameo'd in Norton's The Incredible Hulk during the Abomination fight in Harlem and was deeply affected by that event and brings all that into Luke's storyline in some notable capacity. THAT would have been a payoff 8 years in the making. Also, making Alfre Woodard's MCU and Netflix characters the same person could have been really intersting. Justin Hammer and/or Trevor Slattery at Seagate.
I mean, I think in the end it really doesnt change the end result, but it couldve been cool.
Yeah, it's not about whether or not people would still like Iron Man if there was no Avengers. It's about how awesome Avengers is and a very obvious way to up that ante, combined with a very poor excuse not to.