I'm sure there'll be more now that the idea made it into the games.
I've been thinking of dual type moves also...
For example Muddy Water being Water&Ground, would mean 2x dmg to Fire, 1.5x to Poison, etc. And STAB would only improve it by 25% unless both types matching, giving the usual STAB.
Next, Ice needs to be better defensively; they're way too fragile. Ice Pokemon are hardly used in battle because of how many common attack types they're weak to. Also, they only resist themselves. The problem is that there's no good weakness to remove from Ice, since all the weaknesses (Fire, Fight, Rock, Steel) make sense, since fire melts ice and physical force can break ice. Perhaps adding resistances would be a good start in making Ice Pokemon better. I say making Ice-types resistant to Dragon attacks would be a good start, plus they could be used as a safe switch-in to Dragon-types. Maybe a resistance to Water would work, too; for a long time, I used to think Ice could resist Water, anyway, since ice is just frozen water. If Water can resist Ice, perhaps Ice can resist Water (except Scald; dealing normal damage to Ice-types with Scald would make sense).
There's probably other changes that need to be made to balance out the type system, but I don't want to spend too much time right now trying to figure them all out.
This is my attempt at balancing it a bit. Ive changed it a few times over years.
Initially I didn't wanna make Water weak to poison because, well you can find a reason to make anything weak to poison, and because a Water pokemon is not made of water or even has to live in it, but merely specialized in Water based attacks.
So I kept the Poison reasoning to technique types...and noticed Dark, the only type besides Normal that doesn't have a real elemental ability or strength, it just plays unfair and uses dirty tricks. And faking a strong immune system wont make health issues go away.
But thinking about it again, and how everyone always first thinks of Water as something that should watch out for Poison attacks, maybe that'd make sense after all, and Water is defensively doing great anyways, so bringing it closer to Grass' "league" would be fine.
Poison while walking being removed is good, because now it makes perfect sense that pokemon are in a "frozen" state while inside pokeballs, since nothing else ever seems to affect them and they dont recover or anything.
Not sure about buffing regular poison that way. Turning it into a special will-o-wisp would be overpowered. Although Id love to see a status effect do that, its likely better if its a new one.
Bad poison is just a stronger version of poison, as Toxic spike layers prove.
What I would do instead is perhaps make it lower Sp.Defense? Either way Id apply that to bad poison too then obviously. Would still make regular poison better.
Also, I would make it so that poisoning a poisoned pokemon changes it to badly poisoned.
Frozen status comes to mind... lowered physical defense comes to mind. lol
There needs to be a Hypnosis like move for freezing.
Wonderng what new status effect could be the special equivalent of burn... if possible a type that lacks such moves.
They've tried keeping evolution methods consistent, by never using an old item to give an old pokemon a new evolution. Otherwise Eevee would have evolved by stone everytime, and every trade+item evo could simply be a trade evo, among other cases, all the way to breeding incenses....
Initially Jynx Magmar and Electabuzz were meant as a trio but that never fully made it into the games, and even what was left of that concept was scrapped with GSC, and since then the group has been only going further apart every gen, while Electabuzz and Magmar not just stayed a duo (which they were from the beginning anyways, what with being version exclusives, remember?), but actually got even closer related by new evos.
Jsut getting an evo too wouldn't make it fit into the group suddenly as there are some serious differences, like Jynx being female, having a secondary type, Psychic, and having way lower stats. Also its design doesn't do what the other do: heavily showcase their element. You look at them and go "whoa, a Fire and an Electric pokemon", unlike Jynx which barely anyone would even guess to be an Ice type.
So in short, Smoochum was more like a one-time homage to the scrapped trio concept.