Potential Additional Biomes for existing types

Dark: Dark-type contains species that based off thugs and feral animals, so inner city encounters would make sense for this type.

Outside of that, more species can be made encounterable during nighttime, though this can be done without actually different encounter tables for the time of day, with overworld encounters.

Dark types are already made encounterable in dark forests and caverns, but those biomes are rather new and/or uncommon for the type, so they can be used more.

Steel: There is no dedicated biome for Steel-types. They appear in run-down power plants, caves, mines and mountains, but they have no dedicated places.

Most obvious place for Steel Pokémon would be an iron mine like Iron Island, except you know, good. Similarly, Aron's first place, Granite Cave, can serve as the inspiration for Steel biomes, with large jewel mines containing Steel Pokémon. Iron--rich mountains like Mt. Coronet also make sense.

However, as most Steel-types are magnetic, inner cities can also be good biomes for Steel-types.

Dragon: There is no dedicated biome for Dragon-types, though most of them appear in old shrines or lakes and marshlands, as well as inner mountain water resources. Instead of that, however, more canyon areas can be used for Dragon-types, considering their association with ground, water and flying.

Dragons can also appear in closed off lakes, and underground lakes, as those are the closest thing to a biome they have. More large ancient shrines like Dragonspiral Tower and Sky Tower can be used, but the difference would be that these would now contain no Ghost-types or Psychic-types.