Of course, Korvac would be better saved for an Avengers/ Guardians crossover. He could be set up in the Guardians movie first though.
Korvac would be an unbeatable foe who could bring an epic quality to an Avengers sequel. An adaptation of the Korvac saga, culminating in an all-out battle like "The Hope and the Slaughter" would be awesome. The Guardians could even just be brought in for the final battle.