As for types of plants and trees we need more of in general, I'd put them into 2-3 categories:
1. Temperate and Taiga trees and shrubs (more Birches, Conifers, flowering trees/shrubs, fruit trees and trees with autumnal foliage)
2. Subtropical/Tropical trees and shrubs (Palms, Palmettos, Live Oaks, Ferns etc.)
3. Versatile plants for general ambience/realism (Flowers, Crops, more variety of Grasses and bushes like Raspberry bushes)
We just need a little more variety in general so that each biome/continent combination has a little more variety.