E3 and San Diego ComicCon are really the last two big showcase events before September, right? Besides these it will be Disney-organized PR for DI 2.0.
So, E3 is June 10-12
I'm expecting more information on the toybox expansions and experience trees. Spiderman figure (since Amazing 2 just came out) to show us around the toybox. A villian, probably Loki. Maybe two or three other Marvel characters, to oooh and ahhh over. And if the toybox-only characters from DI2 are getting their own playsets, either by DLC or updates, then E3 is where that announcement is going to be made. I won't expect it after that.
They MIGHT throw in a nostalgic Disney character, probably Stitch if so.
Maleficent is getting her own event, but she'll probably be there for a wider audience to see.
Then San Diego ComicCon is July 24-27, and Guardians of the Galaxy comes out August 1st. So, I don't expect Guardians at E3 - I think it's way more likely they'll show them off at the ComicCon closer to the release of the movie.