the maple and birch trees are still in the game as of 17.2, so is the plains grass. no modding them using uabs needed. they are there, just their icons are missing. you can spawn them in in creative mode without mods. Which gave me the idea that if we could make a suitable ground texture, it could be a go.
adding in other biomes would cause problems for most because those assets are not in the game and i could understand how adding more assets would cause problems. also, rgen is based of the navasgene map.. which is based off a part of arizona.. plains are a logical biome choise.. swamps not so much. but hey.. i'm all for that ocean front property in arizona.. gimme a beach, i'll gag back zombie margaritas