Earlier today saw Disney run advertisements that said something from The Avengers: Age Of Ultron will be shown during Star Wars: Spark of Rebellion that airs tonight.
I didn't see the spot myself, but I was just told about it by our very own Chris "Doc" Bushley who was watching Disney with his daughter earlier today.
Chris said they showed what looked like Cap's shield on some boxes in an alleyway during the spot with mention of The Avengers 2 and Star Wars Rebels.
I don't know if it will be new footage, behind-the-scenes footage or the teaser that was featured a while back on ABC that released the first concept art for The Avengers 2.
I planned on tuning in for Star Wars Rebels tonight as it is, so hopefully we'll get The Avengers 2 as well.
Update: We have confirmation as Disney just aired the spot (we'll have it up in a few; meanwhile, here's a pic)
Update #2: Here is the footage of the spot: