Trees work horribly, at least in my opinion. When the world generates, it uses many wonderful trees, from a big long file of custom trees, which is great. Only problem is, there's only a limited amount of saplings, so once you've cut down some trees, you can't regrow them back.
We need custom saplings per each tree, or a way of regrowing each individual tree type. My suggestion is as follows,
for saplings, instead of using regular ones, would it be possible for you to make "bonsai sapling"
a sapling dropped by a bonsai tree, and regrows a bonsai tree.
I find this a much better solution than regular ones, and by default, an oak sapling doesn't give oak. With this, every tree would be growable and work perfectly.