I've been wanting a tree brush tool, similar to the one in Jurassic World Evolution. As that is another of Frontier's games, I'm hoping they implement that eventually!
It would be perfect if you could filter your brush with both continents and biomes at the same time. Then, you can say paint a North America only temperate forest.
Have to agree a foliage + rocks brush with different filter options (kinda like ZT2) would be great. Right now filling in larger areas around the zoo is really tedious and takes ages, a brush would make things so much easier.