I just ended up making a tree like pic related using SpeedTree, and I imported it into UE4. The shading looks really shitty with this tree, and I have no idea how to into shaders for this game. Is there actually any way to get that "Cell Shade" look in this game? Or is this pretty much a dead end?
Games make use of lodding. The tree will be rendered in its full fidelity only when you stand right next to it. Move away, and if you pay attention, you see it popping to a simpler model. At a very far distance it's just a billboard.
If you're going for that kind of style, you probably want to stylize the model of the tree too. Like, instead of individual leaves, use ellipsoids with tree stuff painted on.
Also, notice that the tree is not subject to cel shading in pic related.
Btw, OP, you making a pony game or something?