Super Smash Bros.
I think this is honestly a decent lineup to start with - still includes plenty of Nintendo's more iconic characters, a couple of "weirdos" (in the modern sense) and overall a decent spread of characters from different eras.
Super Smash Bros. Melee
A fairly conventional lineup given what everyone else seems to be suggesting. A lot of these are fairly obvious secondary character candidates, but then the addition of a couple of oddballs is neat too.
Super Smash Bros. Brawl
Slightly less conventional, I suppose. The remaining characters are a nice mix of some oddballs, secondary character candidates, and a bit of new blood. That said, some picks were difficult to cut - Pokémon Trainer seemed pointless if taking up three spaces, and solo Charizard felt kind of superceded by three other first-gen Pokémon reps. (Admittedly, if I could, I'd probably replace Jiggs with Zard, but that's just the way it is.)
Super Smash Bros. for Nintendo 3DS and Wii U
A lot more difficult than others, so I really took a blade to the roster this time around. Some notable choices - Mii Brawler is the last remaining Mii, Mega Man's cut mostly so Ryu can be "the Capcom rep", and Cloud's cut for reasons that will become apparent. Also, some series - namely Mario and Kid Icarus - are gutted here, mostly because I don't think their characters in this game are all that... necessary. Sorry, Jr.
Super Smash Bros. Ultimate
A lot of difficult cuts here, because there's a lot of characters I'd like to have kept, but felt either not really relevant enough (K. Rool), superceded by other third-parties (Joker) or having their "gimmick" already handled (Terry). The choice to have
Dragon Quest supercede
Final Fantasy was also pretty difficult, but ultimately I think it'll work better. Also, Pyra loses Mythra. Up until now, I've been considering having only one character for each "third-party" company, but Sora's popularity in the ballot just about tipped him over the edge.
Echo Fighters
I'm working on the logic of characters officially considered Echo Fighters, so no clones (Dr. Mario, etc) here. Sorry, Dark Pit.
Newcomers
- Sidon (A popular side character from the Wild subseries of Zelda, generally well-liked by people, appears in all three relevant games, and has moveset potential that'd make him unique to other characters. I know the Champions or Impa are generally preferred, but Impa specifically may have rights issues related to Koei-Tecmo, and the Champions don't appear in Tears of the Kingdom.)
- E.M.M.I (A heavily marketed element of one of Metroid's most popular products outright, as well as having plenty of unique moveset potential. Raven Beak is arguably more popular in Smash circles, but E.M.M.I was the Dread character to get an Amiibo and their own prominent sections in marketing.)
- Magolor (A reoccuring and well-liked character from the Kirby series who fills a role as a playable boss of sorts, has numerous notable roles, and some decently recent prominent appearances in games like Return to Dream Land Deluxe.)
- Ogerpon (A seemingly well-liked Pokémon of a new typing, representing the most recent generation, and actually being a box-art Pokémon for a DLC pack. Additionally, having plenty of moveset potential with its various masks, if you wanted to go in that direction. I think this makes sense.)
- Anna (A reoccuring character in the Fire Emblem series who acts as a fan favourite and a representative of the "trader / merchant" archetype in RPGs, I think Anna would be a well-liked character with plenty of moveset potential. It also helps that she's often an archer-class.)
- Bulborb (I think Pikmin deserves a secondary rep, and it wouldn't feel out of place for one of the series' most iconic enemies - the Bulborb - to get a playable role. We'd essentially make them a really large character with a few other Pikmin enemies in its moveset here and there.)
- Tom Nook (Obviously we all know that one of Nintendo's modern pillars is Animal Crossing, and I think there's totally potential for Tom Nook to have an interesting playstyle - whether it be a puppet fighter with Timmy and Tommy, or using a similar mechanic to Mr. Krabs in NASB2.)
- Mio (Xenoblade Chronicles is a decently prominent series and Mio brings plenty of moveset potential, perhaps with Noah occasionally providing support with Ouroboros attacks and the like. Mio's chosen specifically because of her unique weapon typing compared to the rest of the cast.)
- Octoling (Based on Agent 8's design. Splatoon is iconic, Octoling has plenty of potential, we've all heard this before. I'm going to say they're not an Echo Fighter, rather that they have thematic similarities to Inkling's moveset, akin to K. Rool's thematic similarities to Donkey Kong.)
- MC Horace (I think a "tech demo" rep of some kind makes some sense, and while LABO is generally where people prefer to look, I think MC Horace would be a good candidate for this role. He'd literally fight with Joy-Cons in his hands, mimicking a variety of 1-2-Switch! games. I'll admit I wanted to go with the Clubhouse President here, but this feels more achievable.)
- Rabbid Mario (While Rayman's probably a more popular Ubisoft candidate, I think more directly using the Mario + Rabbids series is a more likely outcome, and Rabbid Mario - as a prominent character from the series with his own unique moveset potential - makes sense in this role.)
- Bub (A representative for Taito overall feels like a really obvious decision to make, and Bub - as the protagonist of both Bubble Bobble and Puzzle Bobble - feels like a natural choice in this regard. I'll note that while Taito are Square-Enix owned, they seem to have their own degree of autonomy.)
Honourable mentions to most mentioned candidates, as well as a character from
Engage, numerous other Gen IX reps, numerous
Kirby reps, the
Wars series (which, in light of continuing global conflicts, will not be added in a playable role), and a
Puyo Puyo character (most likely Arle, but I also considered others.)