Holy fuck. That'd be amazing.
What if Sombra is in the trailer, yeah, but then it's revealed she's also in the BASE GAME.
She's so sneaky and shadowy she avoided being revealed until launch day, and you can play her then.
While I wouldn't personally bet on it, that's more possible than you'd think. On one of the documentary videos, you can see an as-of-yet un-revealed character in the support section:
That picture looks like it could be a low-poly early model (Or perhaps a recolor) of this character:
This picture is found on an open folder in one of the maps, and is generally believed to be Sombra. There's also this picture:
The highlight is mine, and it points out what many people speculate to be Sombra running on the rooftops in the Dorado map. If you can't see it clearly on here, try downloading the image and zooming in with paint.
This picture, incidentally, comes from
this news post on the official website, curiously written in-universe. This news post also says that Soldier:76 might be in Dorado, which is where people are guessing the cinematic will take place.
I personally don't think that she'll be there at launch (I believe that they're probably going to wait until they have a few new heroes done and release them all at once), but I'm almost certain that she'll make an appearance in the cinematic on Sunday.