Terrain hardness

Destructible terrain and mining is the best part of the game and something that can distinguish the game from other factory-like titles.

I would suggest adding more depth to it. An inspiration can come from real life, where different materials are mined by different types of machinery. It would also justify the existence of new machines.

Basically, you generally cannot mine hard materials like limestone or quartz by excavators, as happens currently in game. Instead, you blast those hard ores by explosives and load them by front loaders, since they are already loose enough.

Excavators (or bucket-wheel excavators) can still be good for general purpose non-hard materials. But they are too inefficient for large-scale movement of soft materials like dirt or overburden. For that you would use bulldozers or scrapers in real life.

Such "hardness" mechanic would also feel familiar for a player, as different hardness tiers of raw resources are common in games - e.g. open-word RPG games.