Per wiki policy, Spoilers Off applies here, as pages such as these are meant to be read after consuming the given product, all spoilers are unmarked. You Have Been Warned.
The fact that Dende knows how to power up Shenron, and that it's simply done by dropping water onto its clay model, makes complete sense in context.
Kami only knew the pure basics of the Dragon Balls potential due to living on Earth the majority of his life. Dende, however, had more access to the Dragon Balls' history, studied it and is a prodigy to boot, so he would understandably know the ins and outs of the process. The reason it was never done before was because all the wishes done at that point were nothing that Shenron had not granted before, even before Dende took over, so there was no real point in asking for more.
Furthermore, up to that point the powerhouses were Goku, Gohan and Vegeta. Both Goku and Vegeta would consider that a shortcut and refuse to resort to it, relying more on actual training. Gohan has no similar qualms, and relied on similar power-ups both on Namek and to fight Buu, but didn't need the Dragon Ball in either case since he found people able to unlock that potential.
As for why it needed water, Nameks are shown as only needing water to sustain themselves. They are basically feeding it.
Why did Shenron need a boost in general to grant Piccolo's wish? He's never granted a wish like this specifically. He's granted restored youth, has brought back civilizations and even restored people to their original state, but never granted new power. If anything, his limits was always to grant people power back to the level they were, never more. With this, he now has the ability to unlock it the same way Guru could with Gohan and Krillin.
The mook Piccolo replaced is implied to have some issues with flying. Which is why no one gets suspicious when he does an awful job as a pilot.
Shenron was more able and more willing to give Piccolo something extra, his enhancement by Dende aside, because they are connected as creator and creation, even if Kami's self is mostly subsumed by Piccolo.
It makes sense that Goten and Trunks are very rusty in their fusion dance. They haven't fought properly since the Buu Arc, and the only thing close was their mini fight with Tagoma, and even then it was just a game to them. They haven't even had a proper battle since because not only were their help unnecessary like in the Buu Arc, where they were at one point the only option left, but everyone believed they were too young (which is true) and/or their lack of discipline.
Why does Piccolo's movie star Red Pharmaceuticals as his main enemy? It's the new Piccolo vs. the new Red Ribbon Army, the originals being the two biggest arc villains of the original Dragon Ball!
In a meta sense, it only makes sense the Cell Max is the final boss of the first 3D movie. Pretty much everybody who's ever drawn him in 2D has hated it, thanks to the numerous black spots all over his body being tedious to draw, even for Toriyama. Cell Max uses a model, meaning he's way easier to animate, since they only have to draw all of his dots on his entire body once.
Why do they use Trunks killing Freeza as the example of how a bunch of aliens are fighting over the control of Earth? Especially when other fights in the series were actually broadcast on television? Because Trunks kills Freeza while wearing a jacket with the Capsule Corp logo on it. While the real reason Trunks wears it is probably because Bulma made it for him, an outsider would take it to mean he's some sort of literal Capsule Corp soldier. They never stop to mention it, but it's clear that's the implication.
Magenta theorizes that Capsule Corp is involved in selling the Earth for aliens to colonize, while it shows an image of the Namekians. Technically speaking, that did actually happen - Bulma let the Namekians live in the Capsule Corp building while they were waiting for the Namekian Dragon Balls to recharge to create another Namek after the original one was destroyed. Capsule Corp did actually colonize aliens on Earth for a brief period.
Red Pharmaceuticals handily explains an old plot hole, namely how Gero was able to continue his research without the resources of the Red Ribbon Army... Except he still had them, he was just getting his funds, machines, and parts from the organization that used to fund the Red Ribbon Army and skipping the middleman.
This would also explain their ability to create the Gammas and Cell Max: they had Gero's notes, or at least part of them, they just lacked someone of comparable or even greater intelligence to actually make use of them until Hedo joined.
Normally, Gohan can't see without his glasses, but in Super Saiyan (and above) he suddenly can, which makes Piccolo wonder why. Given that Super Saiyan enhances all the body's attributes, this would naturally extend to blood flow, air circulation and other mundane aspects - unlike past cases of Goku vs Android 19/20 in the Android Saga (accelerating the heart virus) and Gohan vs Basil in the Universe Survival saga (accelerating the poison he'd taken beforehand), it worked in Gohan's favor because he doesn't have any foreign substances in his body, enhancing his sight for the short term.
Gohan uses a Special Beam Cannon at the end of the film. However, it’s not the usual golden yellow color, but more of a whitish blue color. In other words, Gohan uses the attack of one father figure, (Piccolo), but the color is that of his other father figure, (Goku).
Mixed with Development Gag, Cell Max in its semi-perfect form being the strongest iteration of Cell once again follows Toriyama's personal preference for the form, as forthe original manga he was told to hurry up the arc and make Cell reach his perfect form quickly because his editors thought the semi-perfect one looked ugly (later on Toriyama made Cell make his biggest impact in the story by killing Goku in his (reverted) semi-perfect form).
Cell Max becomes immensely more terrifying when you realize one thing: it's only in its semi-perfect form. Assuming it can transform like the original Cell, imagine how much more powerful a Perfect or even Super Perfect Cell Max would be, and that's not even counting how Android 17 and 18 are stronger now then when they were absorbed by the original Cell. So the power boost would likely be even greater.
Even worse, Magenta released Cell Max before he was finished. It's quite possible that, had Cell Max been successfully completed, not only he's going to be in its Perfect Form, but also be sapient, as Dr. Hedo lampshaded he's still working in its brain functions.
It is subtly hinted that Colonel Violet may be Magenta's mother considering that Magenta has a picture of her alongside a picture of his father Commander Red. If this is true than it brings up troubling implications about her relationship with Red considering that 1: He is her superior and 2: She was perfectly willing to rob him blind and abandon him while Goku was tearing up the base.
Did Pan forget how to fly from being too young to remember, or did Toriyama consider the times we saw her fly in Dragon Ball Super noncanon?
While the real answer is more than likely the latter, there is some Truth in Television involved here too; babies do reflexively know how to swim from a very young age before losing the ability growing up, for instance, and while we obviously cannot fly in real life, it makes perfect sense that Pan as a baby could be able to fly on a very similar principle.
Intentional or not, both Piccolo and Krillin's words actually support that, and in the end she ends up unlocking the ability in a sink-or-swim moment acting on instinct. Piccolo also mentions she is a prodigy comparable to Gohan himself, who was shown accomplishing feats by instinct that he could not do consciously without a lot of training.
