I'm fairly sick of alternate forms (i.e. "properly" different with altered stats and whatnot), because I think it's a cheap way of adding something new mechanically. Fusions, Mega Evolutions, Primal Reversions, Alolan forms... These are all less creative than simply making new Pokémon, and it's been so common nowadays that I'm no longer interested. Once you start giving a species multiple forms with different stats or moves or whatever, you start comparing them to each other in terms of their numbers and properties, which I don't think is right.
However, I'm a big fan of Pokémon variants. These are cosmetic-only differences between Pokémon of the same species. Okay, they're just as easy to design as Alolan forms, but they don't try to get away with calling themselves a mechanical difference because they're just cosmetic. That, to me, feels more honest somehow. Variants are solely for pleasure, not to be a vessel for 10 extra Speed points or an Earthquake to give that edge over other Pokémon.
The Pokémon games have done this before. Unown has (currently) 28 variants. Spinda has many many spot patterns to choose from. Vivillon has a bunch of wing patterns (although it loses points in my eyes for only having one pattern be available in any given game). There are gender differences too, like Pikachu's heart-shaped tail and Magikarp's different-coloured whiskers - very minor differences, but they're there. There are more major differences in a couple of species, like Frillish and Pyroar.
You could be simple with variant designs, or you could be more outlandish. Honestly, I think I'd tend to prefer something on the simpler side, things like colours or different stripe patterns, some added or subtracted fur, or altered sizes. They wouldn't spark amazement, but they'd definitely fit in. The larger changes would be more hit and miss, being easier to get wrong for the sake of showing off.
Here are some of the simpler variants I'm fond of:
- Brown Rattata
- Black Jynx
- Different colours of Staryu/Starmie (both the gems and the limbs)
- Upside-down Koffing like in Red/Blue
- Cracked segment Onix
- Terracotta Rhyhorn
And here are some of the more altered variants I've thought of:
- Dual-horned Onix
- Kingler with bits of coral growing on its shell and claws
- A leafier Leafeon
- For that matter, all Eeveelutions with ruffs like the originals have
- Various different shapes of Cryogonal
- Paras/Parasect with different kinds of mushrooms (such as a rare one with Balm Mushrooms)
- Cactus Bulbasaur (the little round ones, but turning into bigger ones as it evolves)
- Ampharos but with wool
And of course, some joke ones:
- Kadabra with a fork
- Tentacool that's shaped like a box
How would variants be used? Well, as I've tweeted recently, I'd like to remove shiny Pokémon (at least in fangames where I don't think they're worth having), and instead have variant Pokémon. Some species wouldn't have variants, some would have just the two (the original and one alternate), and some would have a range of variants. Each variant may be native to a different part of the region (and/or different climates), but I generally wouldn't make them depend on a property of the Pokémon (such as gender).
If you're spriting them, try to go for different poses as well.
