This game sounds amazing.
Would like to recommend MissingNo (Bird/Normal type or Normal/999 type, STAB to none but also without any super effective types) be a "legendary" encounter following the script of the old red/blue (minus the infinite item part). Can imagine how ridiculous those sprites would be :)
Was wondering if you implemented Dual type moves would they also be dual stab bonus? either additive or multiplicative.
Also: if they have 4 types, this game wouldn't be that bad :) Crobat or Golbat and Golem
Such as I'm a Flying/Poison & Ground/Rock:
Weak to:
Electric, Ice, Rock
Ground, Psychic, Fairy
Water, Grass, Ice
Water, Grass, Fighting, Ground, Steel
Resist:
Grass, Fighting, Bug
Grass, Fighting, Poison, Bug, Fairy
Poison, Rock
Normal, Fire, Poison, Flying
With the current single type moveset that would end up as:
Weak to
Electric (immune)
Ice
Ground (immune)
Psychic
Water
Steel
Normal damage:
Rock
Fairy
Fighting
Grass
Resist:
Bug
Poison (also can't be poisoned)
Normal
Fire
Flying
~~~
I think as long as you had some sort of quad type calculator that people could reference that it'd be ok.