Here's how The Avengers saga starting from The 1st Installment will go down from my opinion
The Avengers 2 : New Recruitment - now even though Thanos was hinted to be the villain in the sequel Joss Whedon said for the sequel to be smaller so more of a smaller villlain and since Red Skull is rumoured to reappear from Asgard i choose him and The Masters Of Evil to be the villains of the sequel and the members to join will be
Sam Wilson/Falcon , Hank Pym, Ms. Marvel, and Peter Parker/Spider-Man they become members of The Avengers then the members of The Masters Of Evil should be great matches towards each member and here's who i think should be members The Mandarin, Baron Zemo, Red Hulk, The Enchantress,Crimson Dynamo,Trickshot,Winter Solider,Scott Lang,Dr.Doom, and Venom
the third one should have Ultron and Civil War .
the fourth one well Kang The Conquer should be the villain well that's my saga you guys can comment and help me i still need some members and maybe even some titles to call each movie. so what do you think