Kung Fu Panda 2/WMG

If Shen hadn't massacred the panda village, Po still would have been his nemesis as predicted
In that case Po would have grown up locally, trained by his (apparently) warrior father. Who would have become the Dragon Warrior to defeat Tai Lung isn't known. It might have still managed to be Po, perhaps sent by his father to Oogway to receive more advanced training than he could provide.
 * But then if Po grew up with a trimmed physique like the Five instead of his hefty fat, he wouldn't have been able to defeat Tai Lung, who mastered Oogway and Shifu's pressure-point strikes.
 * Not necessarily, Word of God says that Po in the original film was of a healthy weight for his species. Therefore, it doesn't overstretch the imagination that Oogway would have realized the same thing and tailored his training and diet to reflect that.

Shen has magic involving red light
It explains why the sky turns blood red when he enters his palace, and why his canon fire as well as regular torch fire used by his wolves are blood red in colour as well.

That
It's. In other words, it could be Dreamworks pulling a Gainax at the viewers and screwing with their heads.

After the events of the movie, someone else will invent the cannon anyway, and this time, the Dragon Warrior won't be successful in stopping him.
Assuming this world is an ancient period of time parallel to our own, the cannon will just be invented anyway, since we obviously had cannons in our own world that were eventually replaced with guns. Po was able to stop Shen this time, but maybe one of his descendents, or another different villain, will take up Shen's project again. Of course, in our present time, kung fu is not widely used except for entertainment or personal purposes, so regardless if the next Dragon Warrior is successful, kung fu will live on in some form.
 * Cannons, even awesomely advanced ones built through Shen's genius, aren't really a gamebreaker in the world where kung fu gives you megapowers. Shen managed to use them successully through surprise (plus the most convenient opponent possible), mindscrewing his opponent into failing to react and because the nine masters have severely underestimated his ruthlessness. Still, it didn't took too long to discover the way of neutralizing them through kung fu. Less advanced solution of "not standing and allowing them to shoot you" also worked.
 * It's probable as the conflict was more that Shen wanted to destroy kung fu and conquer as opposed to working side by side.

Ox and Croc will become Shifu's apprentices. They, Po, and The Furious Five will become The Epic Eight.
Why? Rule of Cool, of course. A rule of which Po is one of the most fervent adherents ever seen. You see a glimpse of it when they all (and Shifu) attack together and Po is overcome by the awesomeness of it.
 * But somebody has to be in charge of ruling/protecting Gongmen City, right? With the Peacock family and Master Rhino out of the picture, Ox and Croc are next in line to take charge. They could probably still help out Po and the Five once in a while, but they wouldn't be moving into the Jade Palace any time soon.

Shifu will die in the next film
If Po's father plays a big role in Kung Fu Panda 3 giving Po 3 father figures might make things a little crowded and repetitive. If someone's going to get nudged out, Shifu seems like the most obvious candidate. Dark as Kung fu Panda 2 was, I can't see them killing off Mr Ping as he's probably Po's strongest emotional tie, and after knowing him for 2 films and seeing how he's a lot more goofy and funny than Shifu killing him might be too dark for the film. Shifu on the other hand might be easier to handle. While he's certainly not unlikeable, he's been appropriately solemn and had enough darker moments that would make his death easier to handle.

Another issue is the Ship tease between Po and Tigress. If it goes anywhere Shifu could be a problem, since he's Tigress's adopted father. It may raise squicky overtones if Shifu's around, serving as a father figure to Po as well. Finally, you can only outrun the dreaded Mentor Occupational Hazard for so long. Given how Po's Hero's journey has progressed, Shifu's death looks more and more likely. If Kung Fu Panda 3's tone is as prorpotianlly darker as Kung Fu Panda 2's was to the first film, they might as well call it, Kung Fu Panda 3: Shifu's Doomed.
 * Wha...What...NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!
 * Komodo:' YEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEES!!!!!!!
 * This plot move is unlikely until later films. To have Shifu die would most likely lead to Tigress being raised to Head Nun of the Jade Palace. Not only is Tigress very young for this role, this would also disrupt the strongly suggested Shipping character arc with Po who is even more inexperienced to deal with this kind of development. In short, this is a story development that will likely being used for the end of the 6 film arc, if at all.
 * Actually, they might use it for the fourth film, given that Four Is Death, and all. Then, the remaining two films will be a duology, with Po trying to figure out how to grieve his respected mentor and trying to fight the biggest battle of his life at the same time.
 * It is possible Shifu could die, however, I doubt they'd do it for shipping reasons. We already see the romance between Po and Tigress developing, on top of that, Shifu isn't really a father figure to Po, really just a mentor.

Monkey will actually do something in the sequel
Nah, that's just crazy talk.

Shen Used to Be A Sweet Kid
While not something that's been outright stated by the canon (as of yet), it's somewhat implied by Shen's All There in the Manual backstory. The fandom really seems to be rolling with it, since it would continue in KFP villain tradition.

Lord Shen destroyed multiple villages of black and white animals after his banishment, more than just Pandas.
Though it may be a stretch considering the timeline (this would make Shen impressively older than he looks), one of these villages was home to Snow Leopards - Tai Lung's family. Therefore, Tai Lung was orphaned and sent to Shifu, setting off yet another significant chain of events in Po's life. Nice job breaking it, Villain!

Po's father is going to be the next film villain.
Calling it now...
 * Considering Dreamworks Animation's recent tendency to go all Darker and Edgier, I could kind of see them doing this. Since we know nothing of the father, he could be like anything. Not only that, so far each villain of the films have had their own "color scheme" that would be especially prevelant during an intense scene. (Tai Lung had blue, Shen had red) It's possible green could be the next color, and guess whose father was seen wearing green?
 * Why is it that I can see this happening? Because the next source of drama would be between Po's father figures that's why. Love it when I anwser my own questions.
 * Possibly Jossed. Green is the color of wisdom in the KFP universe and what he wore looks like Grand Master robes.
 * It could turn out to be a case of Light Is Not Good, especailly if the series continues to get darker.
 * Also, wisdom =/= goodness. Maybe this means Po's father has to be an Anti Villain, but he'd still be an antagonist.

Tai Lung will return in at least one of the sequels.
Please? Ian Mc Shane is just so awesome...

Lord Shen found peace in his death.
After his defeat, we see his ship explode into fireworks. It formed a shape of a peacock resembling the villain. It then shifts to the Soothsayer with a smile, affirming this idea.

The next movie will deal with Po having to choose between his adopted family and his Panda family
This movie likes to throw Po for a loop, emotionally. He's just found inner peace, he's secure of his place in the Furious Five, his issues with his stepdad are gone, he's made peace with the murder of his family....and then the next thing, his biological father comes out of nowhere and wants him to come home. And maybe, for whatever reason, Po's father doesn't want anything to do with Shifu or the Furious Five. Maybe he asked Shifu or Oogway for help when Lord Shen was leading an army of wolves into his town, but either help didn't arrive on time, or Oogway knew that the destruction of the village would bring the destined Dragon Warrior into the Valley and chose not to help (which would be Machiavellian as FUCK, and screw with Po's head as well as Shifu's, who might still have some issues of his own with Oogway). In either case, Po will end up facing a choice: between his old life and the life he could live with his 'real' family.

Also there's going to be a female panda in the mix to make things complicated. It's not a proper trilogy if there isn't romance. Besides, the "simple village girl/kung fu hero" wuxia trope is one that hasn't been explored yet in the movies.
 * Well, there is already Ship Tease with Tigress, so it is more likely to go to a Love Triangle direction.

Tigress' Background will feature heavily in Kung Fu Panda 3
Some of the in jokes in Kung Fu Panda was that Po could not possibly be Ping's son, leading to Kung Fu Panda 2 which explains Po's background. Interestingly enough, at several points in Kung Fu Panda 2, Tigress begins to tell her own story only to be interrupted and also shows a much softer side. Ie the proclamation that the hardcore "do understand." If the dark trend of the movies continue, more than likely there will be a focus on Tigress in the next film and a possible betrayal.

There will be a scene at some point where Po dresses up as an elderly grandmother/be confused for one when someone calls out "Po! Po!".
Since 'po po' is the Chinese word for grandmother, it'd be a Bilingual Bonus Visual Pun.

Sinlong will be the villain of the 3rd movie
And he'll use magic attacks against the heroes.

Po's father (the Goose) is actually Badass
Its hard to spot, but even as a baby Po seems to be about as heavy as his father. That the guy even TRIED to give Po goose back rides, speaks volumes. Doesn't seem like the type to give up once mind is made up so one day, perhaps at Kung Fu Panda 3, we'll see him lift up his so Po, a full grown Panda and twirl him happily like a child.

The movie symoblizes the Europeans starting to use gunpowder for weapons
The beginning of the movie talks about the Peacocks using fireworks making people happy, but Shen began using fireworks(gunpowder) as a weapon. The other peacocks represent ancient China, while Shen represents the early-modern Europeans.
 * Shen's white plumage supports this WMG.
 * Could someone flesh this out why white/albino plumage would support this?
 * The Chinese had their own canons. Hell, they even had their own landmines in the BC era. No need to bring unnecessary national identities into this. Its just an age old tale told for thousands of years. Ending of an old age of warfare due to new technology.