I would love to see something like metahuman creator for generating film quality foliage assets to work with nanite specifically. Presuming Nanite will eventually handle subsurface materials. Opacity mapped cards replaced with full 3d leaves and branches? Hell yes.
The leaves, even though high poly, will still need to be opacity mapped for proper detail. You could in theory completely model the silhouette of the leaf with just the polygons at a detail level acceptable even for a closeup, But such detail would require at the very least around 4 000 triangles. Average tree can have around 100-200k leaves.
Let’s assume the tree trunk with branches at sufficient detail will take at least 2M triangles. Modeled leaves with sufficient geo detail to convey the silhouette of the leaf up close on top of that would take about 800M polygons. So you’d end up with 500-900M polygon trees.
Even if nanite could handle them, your project size and packaged game size would be insane, especially given how much foliage variety you need to create natural looking landscapes.
There are alternative approaches such as instancing the leaf meshes on the trunk, but that would probably lead to excessive complexity and performance issues, assuming that you want to be able to generate forests of 1000s of trees and then individually scatter hundreds of 1000s of leaf instances on them.
What I am trying to say that despite nanite being a revolutionary tech, it will still sooner or later need to support at least masked materials to be useful for foliage. You can of course have very simple geometric leaves, with just a few triangles. Some trees are like that on assets meant for offline rendering, but they look really, really bad when you get close to them. And since UE5 is meant for interactive stuff, you can’t really dictate the viewpoint of the player, so you need detail even up close, and tree with opacity mapped leaf cards would totally destroy a tree with simple lowpoly cutout of the leaf shape up close.
I definitely agree that having some sort of Metafoliage would be great. There seems to be some “cartel agreement” in the CG industry, that any plant generation software needs to have horrible UX and be overpriced on top 
It’s just that to me, it’s equally as important for nanite to simply support masked geometry.
Maybe just some special “foliage” mode, where static mesh faces which have assigned masked material would retain their border topology edges and don’t have them decimated.
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Yep. I agree with you on every point. I was originally considering fully modelled leaves, silhouette and all yet you make a good case, despite Nanite’s formidable compression. It really is early days for Nanite, yet hopefully with support for masked geometry, and later on, skinned mesh deformation, perhaps a hyper-detailed forest with leaves blowing in the wind isn’t a decade away!