I like your ideas, but some of them exist already! If I go over each …
Long Fluffies: I don’t think these exist as a sub-species, but super-long fluffies have been depicted in weirdbox content, and it’s funnier if their length isn’t limited. Either way, I imagine they’re better off as a weirdbox thing. I also imagine people wearing them as scarves. Ethical fur scarves.
Gooey Fluffies: Surprisingly, Swiftbitches made something similar to this - slime fluffies! I don’t think they ever caught on though.
Dragon Fluffies: These have been attempted by multiple users on multiple occasions. None have caught on yet though.
Treasure Fluffies: I’m pretty sure I’ve seen sparkly fluffies around, but none that actually have gemstones and precious metals. This has potential, as I imagine a fluffy that has literal gemstones could get into some huge trouble due to its value!
Flutter Fluffies: These are already a thing. Flutterfluffs are butterfly-sized fluffies with butterfly wings that are actually able to fly around! They’re a fairly obscure subspecies, so I don’t blame you for missing them.
Ballin’ Fluffies: Haven’t seen anything like these! They could work, so long as their designs don’t veer into “actual armadillo” territory.
Color Change Fluffies: I haven’t seen these either! I like this idea. I want to imagine they adopt the color of anything they eat though, so they’re always looking for food of a certain color and can starve themselves by refusing to eat yucky-colored foods like kibble, or hide away in shame until their new “bad color” goes away again.
Nighty-Night Fluffies: These guys sound cute, but a bit pointless. Could make for some cute hugbox art, especially if used as a fluffy’s night-light, but not sure they’d catch on.
Sweetie Fluffies: Funnily enough, scented fluffies have been suggested on multiple occasions too. It’s still an underdeveloped idea though.
Giant Fluffies: These exist! The XXL Fluffies are also extra-big, though I haven’t read enough about them to know if anyone’s actually ridden one like a horse. These Giants sound like a mix of Enlarge/Ride-On fluffies and fluffaloes.