Ant Man is not a side character. He is supposed to be a core member of The Avengers. I would add Wasp to that as well.
The Guardians of the Galaxy didnt come in till Phase 2. In Phase 2 they needed to jntroduce the wider Marvel Universe that exists beyond the earth and the GotG was an absolutely brilliant way of doing that. When it comes to DC, Green Lantern should be that vehicle.
And Marvel is still not focusing on side characters aside from the GotG. Black Panther, Dr Strange and Capt Marvel are all core characters. They could do the Inhumans because Blackbolt becomes a major mover and shaker in the Marvel Universe (esp if they plan to introduce The Illuminati). They could do Namor as well. What Marvel is not doing is wasting time on doing a movie about The Wrecking Crew, or giving Elektra a stand alone film.
They had set up the wider marvel univers within the galaxy. At the point where GotG came out (phase 2) the situation on earth had been pretty well established. Thus GotG informed audiences of the political situation in space and showed them exactly how powerful and how feared Thanos was. And it also gave us the necessary back story on the Infinity Gems, so now we can namr them within the other films without needing that exposition. Guardians handled that. It was time.
They dont have to do that with the movies. They have 40+ years of history with most of these books. All they have to do is figure out what version of the characters they want to go with, which stories they want to tell and put it together so it all makes sense. When they started the movie franchise, the first few films might have been trial and error seeing what resonated with audiences. But they are well beyond that now and reaching deep into long established Marvel lore to advance the franchise itself.
Likely somewhere in the middle of those extremes.
We spend as many years as we need to. We need to establish the rules first. Make introductions. Let the audience become familiar with and get comfortable with the characters. Then you destroy their world and change everything. It will resonate with audiences a whole lot more that way. Just look at the box office of the Marvel franchise. They are quite obviously doing something right.
I would have preferred that Antman been a founding member. And I would have liked to have seen Ultron introduced in an Avengers film and be helpful to the Avengers first, then betray them. It would have had much more impact then.
Speaking of Vision. The fact that Jarvis became Vision was awesome. Jarvis was introduced in the original Iron Man and had been in every movie that featured him. Audiences knew and were comfortable with Jarvis. So when Vision spoke with Jarvis voice, the average movie goer feels as if they already know the character. There suddenly isnt this new character that needs history and background in a team movie where there is very little time to do that.
I am acknowledging this approach. I would say this for any studio attempting a long running and wide reaching movie or tv franchise. Not just for comic book movies.