So here's the current line-up for the trading races for Toads:
| Spring | Summer | Autumn |
Level 1 | Beans (weapons/armor) | Piantas (good-aligned pets/tropical fruit) | Toads (clothing and food) |
Level 2 | Bub-Ulbs (good-region fruits) | Nokis (tropical ocean products) | Moles (mining equipment) |
Level 3 | Frogs (special powerups) | Something else... | Doogans (evil-aligned plants/animals) |
Goombas will start attacking between levels 1 and 2, Koopas and Bob-ombs around level 3.
Any ideas of one more race for the missing slot, and what they should carry?
Koopas and Shy Guys will have to fight Yoshis from the start, and will be attacked by Beans in the spring, Piantas in the summer, and Doogans in the autumn, all starting around Level 2 (although they have different appearance requirements, so they might show up in different orders depending on your playing style).
Craws? A bit like Doogans, but rather than animals they have moderately good metals.
How about Star Sprites?
Bowser's Inside Story shows that Star Sprites still have a moderate affiliation with the Mushroom Kingdom. Paper Mario shows that Star Sprites are born the ground and ascend up to Star Haven later on. Star Sprites can have infinite life spans and take a long time to grow up, but when they have kids it can be in groups of seven. They would have to be live in good-aligned biomes to keep the mystic quality they are supposed to have.
They could trade Star Pieces, an item made to be melted into a very rare metal or be a stone type that the Toads can work with. I think stone would be better and the Sprites could also possibly bring weapons or armor made from a metal that can only come from the Star Sprites. I like this because Toads can't really dig for the deepest metal, so giving them some kind of long term reward of this type seems like a nice thing.)
They could also trade other Star themed goodies even maybe trade young Star Sprites to act as a support pet. (Even better if it is possible for Toads to make wishes on the Star Sprites to give them power boosts, can interactions target allies? It seems like you can have it target specifically Star Sprites from the way the tokens look.) The other Star themed goodies could be like valuable plants that the player can try to grow or maybe a neat fabric or something.
And a Star Bit item that can be turn into a Star Cake.
On that note, Lumas could be a secondary caste for Star Sprites as the creatures are nearly identical anyway. (I know you like to stay away from castes, but here it is because really they could easily just be the same species with different names and they would hang out in the same place. This way they form societies together and live in Star Organism harmony.)
Edit: You could also add an interaction in good aligned biomes that causes it to rain Star Bits, Falling Stars, or Star dust or something. It would be neat bit of flavor for those areas.