In 2012 I don't think any of us ever thought there was any chance of Marvel ever doing a space comedy with a bunch of idiot no-names like the Guardians of the Galaxy, yet here we are.
They did a sit-com for Wanda, I wouldn't rule out a musical. I mean, it's not something I'd expect to see or anything, but never say never, just when you think you know how these guys are gonna play things, they switch it up on you.
....probably not, like, a whole musical movie....but maybe an episode?
As for the Iron Man 3/ Dark World thing....pretty sure IM3 comes first; it came out in theaters first, which is *usually* indicative of timeline order, and Thor had spent a unspecified but substantial amount of time tamping down the rebellions across the Realms in Dark World, while in IM3 Tony is still dealing with the very fresh trauma of the Chitauri invasion.
But both films stand on their own with no real connective tissue between them so it doesn't matter which you watch first.