Some quick scene blocking, some models are place holders.
Placing trees one by one when there is no quick paint objects 
I’s a small graphic capabilities demo (no game) , Armory materials and shaders (foliage transluency, parallax, refraction, pbr, detail texture, global illumination, emissive materials, post effects).
I’ve never used it, but there is a grease pencil scatter plugin for blender. It is supposed to help you spawn objects on another with grease pencil strokes. That might be useful for placing trees and stuff like that.
Can’t wait to see the scene with materials. 
Reading posts, it looks limited to a plane only.
https://blenderartists.org/t/grease-scatter-addon/515864/10
It’s one object after another, material painting takes lot of time unfortunately, and i’m doing it in my spare time.
Oh, BTW that video you posted earlier about a Substance Painter type workflow in Blender was great. I’ve wanted to do that for a while now. I knew how to do the shader mixing, I just couldn’t figure out how to see what I was doing while I was painting. It’s all in that opacity slider.
So how do you go about the painting? Like do you use Blender or something else? And how do you get the differen maps such as roughness, color, normal, and occlusion. Or do you get it from somebody else?
I use Substance Painter to work fast, it can bake normal, curvature, id color maps,ao and other maps.
You export your maps once you finished painting.
I use a custom script that copies and replace changed models and textures from a working directory to the game engine assets folder (both have same structure folders), it’s just one click to synchronize my work when i want.
What size texture are you using?
Textures are 2048*2048, it could be 4096 for big assets on LOD 0 depending on game resources budget.
A new model addition, a power tower system (the demo scene will have some light alien or sci fi vibe).
Design change
About your problem to place many objects easily
I personnaly use https://blendermarket.com/products/asset-sketcher-v1
Not free, 30$, if you know where to look, you will find torrent sites, but I don’t apologize for piracy so buy the product to support the creator, it is a great addon.
I’ve never had any problem paying for open-source software if it means that the developer can actually afford to finish the thing and to give it the support that it deserves. It’s no fun to put out a great thing, then suddenly get all kinds of requests to support and to enhance it and you realize that no one’s paying you a dime. That’s just not fair, really. Time is … Money … and someone who’s willing to take the Time ought to get some Money in return. We all stand on the shoulders of giants. Who needs to be faced with “abandon-ware” because the developer, metaphorically speaking, “starved to death?”
Some modifications and new colors ( i got better results with 3D coat than Substance Painter).
I must say that I’m not sure what the additional details would need to be. Looks good to me from all sides.
Some more details to make it more interesting, also make the lower part stronger.
A quick sketching
