*Long Post

Actually, I'm just gonna post my crackpot and potentially dead-freaking-wrong CSI posts. Gathered from prior threads with some new musings. Be warned this is all just educated guesswork. This could all be completely, utterly, and embarrassingly wrong.
Why would she be included?
By now I think it's pretty clear that the woman in the trailer is indeed Li Mei as I and many others suspected. She is shown in close up and has actual storyline related dialogue. It's implied that her information is what tips Raiden and the Scooby Doo gang that the Civil War in Outworld is getting out of hand and that the prize is the amulet.
Why would she be playable?
She has literally the same amount of screen time and dialogue as heavy hitters such as Sub Zero; she actually gets more dialogue than the new characters that were obviously revealed. It wasn't a "blink-and-you'll-miss-it" cameo but full on highlighting. She appears to be a catalyst for the Scooby Doo gangs' adventure. Having her exist as only a random NPC cameo with no build up to just randomly show up and say, "Oh hai guys the war is getting totes cray-cray up in Outworld lolol" makes no sense; she would only be noticed by series fans. I find the following situation to be more likely based upon the story structure of MKvsDC, MK9, and Injustice:
The amulet Mileena covets, for whatever reason, has found its way into Li Mei's village. Mileena's forces march onto the village. Kotal's forces take notice and also move on the village to confront Mileena and co. Battle breaks out in the village. Li Mei then does what she did prior to MKDA (defend the village, nearly defeated Kano before he got help) and defends the escaping villagers and refugees. This consists of a couple fights with punching bag characters on both sides before she is forced to leave. Li Mei and other refugees to seek shelter in Earthrealm where she proceeds to spill the beans to Raiden and co. in hopes they can intervene and stop the conflict before more people are hurt.
This scenario would be similar to the bit-part chapters that characters such as Jade had in MK9. How it might actually play out might be similar to the role that Stryker played during his chapter in MK9.
But I still think she's just a cameo...
I concede that that is possible. There is a problem though. This battle was a big deal from her words. It was a big deal, presumably, due to the amount of damage it caused and because it was fought over one of the most powerful trinkets in MK lore. This would not be something that is quickly glossed over in some anti-climatic exposition in an interrogation room. This (to me) is a battle that must be shown or depicted in some regard. It's not enough though to just show it though is it? Nobody that's playing this game is going to want to watch characters fighting; they're going to want to be fighting.
Also keep in mind Li Mei's character. In MKDA she knew how to fight extremely well; in the Konquest mode it's said she defended her village from the DA's forces and actually nearly defeated Kano until their bout was interrupted. This is the actual quote from that game:
"Although quite young, Li Mei grew up in the presence of martial arts masters and can hold her own in a fight. When Kano's troops invaded her homeland under the banner of the Deadly Alliance, she fought bravely and bested many of Kano's men. She might have defeated Kano himself had Quan Chi not interfered.
This is before she trained with Shujinko. Before she trained with Bo Rai Cho.
In terms of storyline now think about this: is Li Mei really going to just let her village and her people get stomped? No way. That completely goes against her established character traits. She wouldn't be some observer in the background; she'd be in the thick of it trying to protect her people from both sides. This fact alone makes Li Mei a candidate to be a playable character (perhaps in flashback as MKX's story is said to be told in) to experience this battle and more importantly set up the next chapter in the story. She's present for both. She's the story link.
Any other circumstantial evidence?
Sure. People are confused by what Li Mei is wearing. They for some reason assume she's wearing some kind of SF uniform. She's not.


As we can see she's wearing some type of collared pseudo Chinese/Outworldish top with a mix of different materials and fabrics. Leather, and a kind of silk like fabric. The silk portion has floral patterns which is a motif Li Mei had with her MKDA primary.
But that doesn't seem like something she'd wear...
I somewhat agree. Though she is clearly no stranger to the traditional MK bikini/leotard fetish she does wear clothing that is far more conservative than traditional MK females. Her MKDA primary and MKD/A alternate being examples.
Now with NRS's obvious attempt to somewhat mellow out their female designs what she's wearing actually does make more sense. This seems like perhaps it might be something similar to this:

Makes sense right? Now imagine if you will what her footwear might look like. Might it look something like this?:

The foot/leg definitely looks smaller, slimmer and feminine...looks like it might connect with the outfit...could be?
TL;DR - basically there's a fuck ton of circumstantial evidence that could potentially suggest Li Mei is playable if you're willing to entertain the idea. However it's all circumstantial evidence and as such could be completely misleading and incorrect. Believe at your own risk.