Give Sif a larger role
Introduce a good villain who isn't LokiI like Loki as much as the next fan, but so far, the other villains have sucked. Laufey is easy to forget and Malekith ended up rather flat. Bring in a baddie we can love (or love to hate).
Let Jane be awesome
Something like Dark World climax cool. Just let her be the superwoman of science and actively help Thor and all the main cast.
Tell us what Thanos is planning to do
When is he going to come for his price from Loki? Just give us some idea of what's going on here.
Give Selvig some respect
One of the things I hated about Dark World was what it did to poor Erik Selvig. The man was forced to do the bidding of Loki. He deserves better than being reduced to the comic relief.
Address what happened to Odin
Okay, is he dead? If so, did Loki kill him? Is he imprisoned somewhere? What happened? Come on, tell us!