Let the speculation begin. Per those tweets it's not the trailer that has them all excited. The only things I could think it could be might be:
1. Some sort of character rights/movie announcement, like X-Men or MAYBE F4 to the MCU
2. Same thing but with Spidey, but he's already MCU so that doesn't seem big enough
3. The announcement that the next phase is Secret Wars, building on what the Russo Bros said this week
4. A new movie to the slate that is totally unexpected. Daredevil?
5. Maybe it's trailer related, but we're getting more than a trailer. Like some sort of supercut feature of the MCU? Doesn't seem big enough.
Seriously the only thing that seems like it could be "bigger than what you think it is" would be an X-Men MCU announcement, but that just doesn't make sense given the fact that Dark Phoenix isn't even out.