there currently isn't a way to change within which biomes the naturally generating waystones can generate. as they spawn exclusively within vanilla biomes, i'm pretty sure that internally this mod uses a tag to pick the biomes, but the tag is presumably one that already exists, such as #minecraft:is_jungle for example (there are worldgen tags too, but as far as i know they aren't used for this). it'd be annoying to have to change a vanilla tag just for this mod, so i propose the creation and use of tags that are specifically created by the mod (a tag that would define the biomes within which the sandy waystones spawn could be named #waystones:sandy_waystone_spawns for example). this hand over the much needed and appreciated control over this feature to modpack makers such as myself!