I'm not sure if this suggestion already exists, but I couldn't find it on the website.
I think it would be really cool if we would have the ability to create two-high plants like sunflowers and also give them two different textures. (top and bottom)
Also, I believe it would be really cool if we could give the the top 'block' of growing plants a custom texture (like bamboo and kelp)
Finally, it would be cool to give the growable plants an 'inverted' option, like those red twisting vines from the Nether Update (1.16). This would make the growing plants grow downwards and they would spawn underneath blocks.
I think this would fit in really well with the new update, because the spawning of custom plants in custom biomes/dimension are already improved, so these improvements would fit with that :D