So, I'm playing as the Elves on Veteran difficulty. As I mentioned before, it would be nice to add some variety to the environment; right now, it's just dark green, monotonous grass everywhere. The heroes are developing very quickly; I think soon, auto-battles will be able to resolve everything. Therefore, it wouldn't hurt to make neutral creatures much stronger further away from the castles. For example, two powerful artifacts – a bow and a quiver – are guarded by 9 Colossi, which a secondary hero easily defeated. On Recruit difficulty, there would be 2-3 Colossi there. Don't be afraid to make the neutral creatures stronger. Regarding the Silvan trees, they aren't needed near the castles (it's better to level up a hero from level 38 to 39 than from 15 to 16). So, it's better to place them on neutral territory, if there is any.
The cities are interestingly located in relation to each other, but it would be nice to add some kind of road between them (this relates to the issue of adding more visual variety). And also, add more uneven terrain. For now, that's all. While exploring my territory, a lot of time passed, even with efficient logistics. A couple of stables would also be helpful.

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Oh, and one more thing. Make the shore look like a shore, i.e., with rounded or steep outlines, sand at the border with the water, etc.