When I first read the concept of Invasion on Bnet, I thought "Hmm, this should be fun." I saw the Reach Beta trailer and saw the Spartans and Elites charging towards each other and immediately said "I need this." I love the concept of Invasion, and I loved every second I played it. The different gametypes and dynamics and teamwork crammed in the on gametype was amazing.
Yes, i would love to see Invasion back. We can do tiered Extraction, but I doubt that'll replace Invasion. I don't know if it's possible to bring Elites back into multiplayer as a separate character model, but I'm leaning towards the no side of things.