If you notice, most of the character demonstrations has SpongeBob as the punching bag to demonstrate the characters with. Makes sense, since he's a massive Stone Wall in his series, and a No-Holds-Barred Beatdown by Flats Flounder felt like tickling to him, so he'd be ideal as a punching bag, or better yet, a Damage Sponge.
On a surface level, it may seem a strange choice to have two Avatars as playable characters, since they technically have access to the same abilities. But as it turns out, neither Aang nor Korra play remotely similar to each other. That's because while they do both have access to all elements, both their methods learning them and their fighting styles directly oppose each other. Aang was an airbender long before he even started learning the other 3 elements, still predominantly used airbending even after learning other elements◊, and his pacifistic nature leads him to prefer evasiveness, speed, and confusion tactics, only resorting to more powerful elements when necessary (in this case, calling the Avatar State). Korra, on the other hand, was naturally adept with water, earth, and fire since she was little, not favoring any one in particular, and approaches problems with a more headstrong and aggressive attitude, equating to her charging energy into her moves before striking.
Why is Reptar representing Rugrats? Easy, this is a fighting game and it would look very out of place for a baby to get into fistcuffs against Aang or Danny Phantom (the oldest main characters of the series would be Angelica and Susie and they are still only three).note As revealed in What Could Have Been, Angelica was in consideration to be a part of the playable roster for the first game.
Why is Oblina a fighter instead of Ickis or Krumm? She's the best scaring student out of the three, and thus, she's the best choice to go out and scare the other fighters.
Why Helga instead of Arnold? Like Aang, Arnold is a pacifist. Unlike Aang, Arnold never fights once in his home series; even when Harold challenges him, Arnold wins by not fighting. Helga, on the other hand, is very much prone to violence.
Why is Glove World Sandy's stage instead of Patrick's? Because Patrick is afraid of the glove mascot!
Why was Garfield revealed on a Tuesday? He hates Mondays.
The Character page here mentions how Lincoln and Lucy are the only Louds in the roster, and are foils to each other. Lincoln's an optimistic and energetic kid (admittedly with a few issues) while Lucy is usually gloomy and obsessed with the macabre (though she isn't above optimism). This may also be the case with Leonardo and Michelangelo. Leo is the level-headed leader and arguably takes his ninja training the most seriously out of all his brothers (yet he isn't above having fun with the others), while Mikey is the light-hearted party guy of the group (but can get serious if someone is in danger).
At first, Aang eating Krabby Patties despite being a vegetarian seems like Gameplay and Story Segregation, but it actually makes sense; Krabby Patties are plant-based, and contain no meat. This is also why Zim, who's extremely allergic to meat, is fine with eating them.
It's probably not a coincidence that SpongeBob and Lincoln are the most well-rounded characters. Both characters are primarily defined as The Everyman and can easily adapt to anything their show needs them to (SpongeBob via Rule of Funny, Lincoln by constantly dealing with his sisters' antics).
It may seem weird that Jenny's reveal trailer had Reptar as the bad guy rather than Zim or the Shredder, but consider that, while he's not evil and he's good with kids, Reptar is still a giant Not Zilla that wrecks buildings left and right, and Jenny commonly fights giant monsters that threaten Tremorton. Reptar was doing what he usually does, and Jenny was called in to stop him. There's also the fact that Reptar is from a show about infants, while Jenny is from a show about teenagers.
In his rampage, Reptar is shown tossing Zim off a building. Considering Reptar being an aforementioned Destructive Saviour, it's possible he started rampaging while fighting Zim, and Jenny's Late to the Tragedy. The bad guy has already been beaten, now to deal with the angry Kaiju that beat him.
Timmy Turner, despite his home series having been one of Nickelodeon's biggest cartoons, has a valid in-universe explanation for his lack of presence: Da Rules forbids a godchild from making wishes to win competitions. Otherwise, Timmy, Cosmo, and Wanda would mop the floor with a good amount of the roster a hundred times over.
In 2, SpongeBob's new down light attack has him slamming the ground with his anchor arms. Despite the attack looking fairly tough and forceful, it's actually very quick and doesn't deal much damage. Considering the anchor arms are basically balloons with no real strength or lifting power of their own, that's likely why they're used in his down light and not his down strong.
A scene in Grandma Gertie's trailer has her fighting Jimmy Neutron. It becomes more hilarious when you remember that one of Jimmy's enemies is an alien who takes the form of an elderly woman.
She’s also shown fighting Raphael and Donatello who also had a bad experience with a Crazy old lady.
Mr. Krabs being a Paid DLC character in 2 is very fitting since Krabs is always expecting people to spend money.
When talking to Master Splinter, Raphael namedrops Leonardo and Michelangelo but not Donatello. Aside from being playable, there is a possibility that Raphael and Donatello were the only ones already dealing with Vlad's takeover of the multiverse right before getting trapped in the Timeless Stardial. Raph would most likely have already run into Donatello in his mind-controlled state.
◊, and his pacifistic nature leads him to prefer evasiveness, speed, and confusion tactics, only resorting to more powerful elements when necessary (in this case, calling the Avatar State). Korra, on the other hand, was naturally adept with water, earth, and fire since she was little, not favoring any one in particular, and approaches problems with a more headstrong and aggressive attitude, equating to her charging energy into her moves before striking.
