Serpent Pokemon: Okay, cheating a bit, but I could see this being a fun series of plush of the long serpent Pokemon (though not TOO long obviously, like we're not gonna do Onix or Milotic here). Thinking Ekans, Arbok, Dratini, Dunsparce, Seviper, Huntail, Gorebyss, Serperior(?), Silicobra, Sandaconda, & Dreepy.
Persian & Arcanine: While riding on Arcanine's back had already been an established thing, Let's Go also revealed a big enough Persian can provide their trainer a lift too. And it looks fun, who wouldn't want to ride on a back of a lion-dog or pseudo jungle cat? Yeah I could include the horse Pokemon and Legendary Beasts too but I think these two would probably be the best choices overall.
Victreebel: Can you honestly say no to being eaten by a giant pitcher plant? James approves:
Though if you're a coward I guess you could also put other stuff in it like other (life size) Pokemon Plush.
Grimer/Muk: I mean as long as they're not made of poison sludge they look like fun to use as a beanbag chair.
Electrode:
Skillz.
Sudowoodo: They made a Snorlax so why not the other Pokemon that's been used as a roadblock? Or you could put it in a plant pot and place it among your other potted plants as a joke.
Wobbuffett: Make it like an actual punching bag which comes back up after punching it.
Delibird: Makes for a decent Santa substitute, especially if they make its tail into an actual bag it can hold presents in.
Mantine: Surfs up!... Okay, you probably wouldn't use it in the water... actually, wait, this would make for a better pool toy or bodyboard ideas. SCRATCH THIS!
Donphan: If they can find a way to have it roll up and keep rolled up I think it could be fun to use as an overkill bowling ball or just roll it at each other.
Hitmontop: Probably too difficulty of an idea, but same idea above except spinning on its head top.
Miltan... NO!
Altaria: Looks SO fluffy.
Absol: Like, there's no reason to make a life-size Lucario aside from the cool factor, so here's another Pokemon to do it with for no reason aside that it looks cool.
Latias/Latios: I try to ignore including bigger and more complex looking Pokemon, especially ones just because you want to ride on them, but I just had to include these. Or maybe you can get away with their Mega Form and just include eye patches to change their eye color depending on whether you want it to be a Latias or Latios.
Bibarel: Memes (can't do Bidoof as all Bidoof plush are essentially life-size).
Spiritomb: If they could have it light up it would make for a great Halloween decoration.
Carnivine: Not as fun as Victreebel BUT same energy.
Rotom Appliance Formes: Just cause I think making a plush out of a haunted microwave, washing machine, fridge, fan, and lawnmower is a funny image.
Arceus: I mean, it it Pokemon God. Not likely, but had to mention it.
Whimsicott: SO FLUFFY.
Klink family: It's pretty much 3 Pokemon in one, like make it attach via magnets and it can be a fun construction toy (maybe throw in some extra parts so they can make their own Gear Pokemon machination).
Chandelure: Another fun Halloween decoration if they can get it to light up.
Cryogonal: Just think it would be funny if one of the life size plush was just literally a giant snowflake.
Furfrou: If they found a way to allow it to be customizable to its various Trims could be a fun toy to personalize.
Honedge family: They're either a single sword, two swords, or a sword and shield, what kid wouldn't want that (especially if they got siblings to swing them at)?
Vikavolt: Especially if they make it so you can load up something like disks between its mandibles and fire them.
Toxapex: Not only to give competitive battlers a heartache upon seeing it but if they made its flaps big enough could make for a personal tent.
Shiinotic: Make it light up and use it as a lamp.
Palossand: Similar idea to Toxapex minus scaring competitive battlers, make it hallow and you have your own permanent sand
castle palace.
Dhelmise: FUN FACT, Dhelmise is 12 feet tall, because it's a freakin' boat anchor. Do it Pokemon Company.
Nihilego: I mean, it's literally designed to fit over your head like a hat. Come on, put it on, what's the worse that can happen...
Magearna: Could be a fun one if they can get it to roll up into a Poke Ball.
Eldegoss & Dubwool: F. L. U. F. F.
Rolycoly & Carkoal: I'm just curious if they could make the wheels spin.
Cramorant: And give it Gulp Missile action (comes with a Arrokuda, but you can fit other small plush in to also fire them)!
Alcremie: I can see Alcremie being one of those plush that come with sets of fabric safe markers that you then use to make it into any flavor you want. Also comes with accessories to use as the candy decorations.
Falinks: You made a Pokemon that was six individual creatures, owe up to it and make a six-part plush (which I guess you could have also done with Exeggcute, but its funnier with these guys).