Ideas and suggestions.

Here's an idea to make the Earth skill less overpowered and essential for winning: change the teleportation mechanic. So, a hero without the Earth skill would teleport to the nearest town, but if that town is occupied, they would teleport to another nearby, unoccupied town, and so on. Naturally, when you have 10+ towns and need to reach the farthest one, it will be more difficult and take longer, but this way, you can still play reasonably well without the Earth skill, and it will allow for more strategic maneuvering.