So why don't we talk possible Assist Trophies? I really hope we have an absolute ton of Assist Trophies this time, because they're a way to get boatloads of characters in without having to worry too much about balance. Some can even have reused functionality.
Lets do this, i'll do one for each series that is currently in the SSB series with playable reps (plus AC just because), odds are guys from your extensive list will pop up here but i'll detail what they could do.
Mario series: Bowser Jr
He turns up and uses the Paintbrush from sunshine to spread goop across an area of the stage, tripping increases to high levels on the goop itself (like the Tingle assist with the bananas in Brawl) and causes slow poison like damage. When I say tripping don't see this as me saying that "oh boy lets have tripping in SSB4 as well!" the random stuff would be removed, low attacks, banana peels and maybe Ice should still cause the effect itself.
DK Series: Cranky Kong
Slowly staggers around mumbling to himself for a period of time, will strike you with his cane if you get close enough, the trick is that the cane will either do minor damage with minimal knockback or have the power of a homerun bat.
Zelda Series: Majora's Mask
Okay get ready for gimmickry, the mask itself pops up and positions itself centre stage and starts bringing down the moon which will instant kill everyone on the stage if it hits, players will have to stop fighting and temporarily join forces to punch the mask to pieces and save themselves from the calamity. Or you can be a dick and defend the mask because you like trolling your friends.
Metroid Series: Mother Brain
Sits in its jar/tube shooting eyeball projectiles, so far so basic, just don't think of knocking the jar over while its out there or else it will rise up in the dinosaur style form from Super Metroid and wreck up the place with overpowered beams. And no the person who gets MB can't break the jar themselves, that would be too easy.
Pokemon Series: Barry
Usually i'd skip pokemon since it has pokeballs but then the idea of Barry running around hyper actively bumping into people was too amusing to pass up, causes no actual damage but has a bumper like knockback when he runs into you, the kid needs to calm down once in a while.
Yoshi Series: Pak E Derm
Stolen from your list RealDeadOne but I loved the idea, an annoying assist trophy that blocks you from bypassing the immediate area around him unless you smash him onto his backside, projectiles can still pass through his invisible barrier space leading to craftiness from projectile users. Also the summoner has the ability to bypass Pak E Derm for greater annoyer potential.
Alternative series idea would involve Shyguys because they are awesome, possibly the limbo ones from Yoshi's Story that trip people with the moving limbo poles.
Wario Series: Captain Syrup
Think about how Munchlax likes to run over to items and eat them, well Syrup would run over to items to steal them, except she would have no hesitation to wallop anyone in the way of her "treasure" so to speak. Could have some light damage dealing black sugar gang spearmen trailing her because why the hell not.
Kirby Series: Scarfy
If you've played a Kirby game you no doubt know about Scarfy, it may look pleasant but go near it and it will chase you down suicidally with a bang, it would pretty much work like it does in the games.
Alternatively the Captain Syrup idea could work for Daroach freeing up a different Wario AT.
Kid Icarus: Pyrrhon
"Pyrrhon has a delivery and the return address is justice!" He flies around, blasts folks with fire while making quips and boasting, basic but gets the job done.
Mother/Earthbound: FrightBot
Joke AT of the game, The Fright Bot will tell you stories so terrifying you'll not be able to go the bathroom at night ever again and various other tales, basically it's a Resetti but you can halt it by destroying the box itself.
Admittedly I could do a lot better here but it's been too long since I last played Mother 3 and i've never played Earthbound so yeah.
Starfox: ROB64
Location confirmed: sending supplies, and the supplies are really good ones at that, like a smash ball or health recovery, or of course a smart bomb. Sent straight to the player who gets ROB64. ROB doesn't actually show up himself, he sticks to his textbox.
Pikmin: Swooping Snitchbug
Swoops in, picks up a player or two and either dumps them into the floor trapping them in a pitfall/DK headbutt state or just off the stage entirely like the bastard that he is.
Fire Emblem: Black Knight
Personal bias towards this guy compels me to have him as the AT, he walks in starts beating the hell out of everyone, final attack involves warp powder teleporting making it like Lyn's critical hit from Brawl.
F-Zero: Pico
He's an ex hitman if my F-Zero trivia is correct and one of the original characters to boot. He selects one target out of the players available and pursues them alone until he times out, is proficient with ranged and melee attacks.
Animal Crossing: BEES
Beehive drops down, you know the drill.
Sonic: Eggman
Why we didn't get him in Brawl I have no idea. Eggman shows up in his trademark pod thing, he'll then pull out one of his numerous boss battle tricks, say that there is 4 possible outcomes here, one would be the original wrecking ball for example, the other three options would have to be ones that involved his standard pod as opposed to his more elaborate machines. Also the music changes to his Sonic 2 or 3 boss theme while he's out just because.
Metal Gear Solid:
a copy of MGS4
You pick it up and either gain a massive damage recovery and temp invincibility because you feel it was the best game ever ever ever or you take tons of damage and suffer a brief poison effect because you feel it was an abomination
Errrr.....
Psycho Mantis comes out, and basically does the Manaphy heart swap thing from Brawl but for every player temporarily while reading your save data, controller swapping cannot save you.
Leaving out Game and Watch because every possible idea is either in his moveset or the flat zone stages. And avoiding ROB because he has bugger all aside from like professor hector.