For example there is Waterdude's Wishing Piece which can summon a random legendary, seemingly regardless of the environment the den is in, that could perhaps be instead achieved by setting an NBT value like the recently updated pokeballs that references its own list of curated possible den spawns e.g. "data/pixelmon/raids/only_legend_raids.json". perhaps with its own unique drops file & npc trainer data to accompany it so more difficult challenges can be set & rewarded accordingly. This could again borrow inspiration from the pokeballs and be able to set what the wishing piece visually looks like to the player by texture such as alternate colour and/or shape to offer some distinction between pieces and their effects. Or perhaps alternatively store this data inside the den entity itself like a shopkeeper does with an edit tool to swap what possible raid configurations it can roll, existing and new dens could convert to this system by defaulting to using whatever biome they load in to if not already set.
By upgrading from an all in one file, and possibly away from any-den vs only-legends-den via wishing pieces, more tailored content can be created and distributed as datapacks, free of biome restrictions and more in the hands of players to expand on.