For me, the primary appeal of the first two films in the series was the irreverent tone. Why take the third installment so seriously? (Or at least, advertise it as such).
It's a sign of growth.
Doing the same things but bigger(Iron Man 2) ultimately blew up in Marvel's faces
If part 3 is
-Big intro by Iron Man/Stark
-Stark goes back to his garage and tinker's around
-Stark emerges from his garage with a new toy and beats bad guy.
Then Marvel would just be spinning its wheels and going nowhere in character development.
To make these movies work you need expand it's universe.
There are ramifications of the "Avengers" NY battle. We're going to see them