What i'd really like to see are Mourning Willows... don't know the exact english name, the latin one is Salix babylonica. These trees are often seen at lake and river sides, easing up the tree line. What's also missing are smaller varieties between bushes and trees. These plants do not have prominent tree trunks, but are up to 3 metres high, like elderberry bushes or lilacs. If you try to create a forest, you need to look at the forest line and how the plants look like there. If not, it'll look goofy... like sticks in the earthen surface. The plants at the forest frontier, neighboring a field for example are at first simple, small bushes, which grow up to a height of 1.2 to 1.5 metres. We already have those in the game already. But the next ones are plants/bushes growing as high as 2 to 3.5 metres, depending on the region you look at. For example brambles, raspberry or the above mentioned elderberries/lilacs. Regardless if you create a river or a field, you can't put just some trees neighbouring a flat surface. A forest doesn't start out of nowhere with 200 years old oaktrees
That happens only if some humans have cut away all the other things in the way. like in parks. But this looks artificial.
What's missing is not the general variety of plants even if i mentioned some, but the limited size an form. It'd be a great help if there would be an opportunity to grow or shrink objects like trees/plants. It's maybe the easier way to add some more freedom. A simplet black pine model can fill a whole forest, if it's not always the same height
I've done this before in Skyrim modding, and they do not have more tree models. They just use different sizes and rotate them. Did you know that the whole Rifton area has roughly only 5-6 tree models? And these are birches, not the more simple conifer like trees.
edit: A nice addition would be also a colorable climbing plant
