Being able to spawn in any arbitrary block i.e not just blocks added via greenery itself would allow for A LOT of new functionality fo pack makers. As an example, most modded plants cant be spawned in via bonemealing grass. But if greenery supported adding those modded plants to the terrain spawning and bonemeal spawning, there would be a lot more consistency in packs that make use of those plants. i.e modded plants could be bonemealed in the same way that vanilla and greenery plants are. AND because some very popular mods dont respect structure rules for superflat maps, it would fix the unwanted spawning of plants on superflat maps. eg. Plants and Project Vibrant Journeys are two example mods that are very popular that ignore the plant/structure spawning rules of superflat that could be fixed in this way. This functionality would also allow for spawning saplings, vanilla wet sponges, mushrooms, cacti and other plants and fungi that can't currently be added or spawned in. Mods like biomes o plenty that have no option to disable its own grass without disabling vanilla grass as well would be fixable by using greenery to spawn in vanilla grass without unwanted grass types along with it.