I imagine upgrades costing mixed resources, like 8m gold, 5m elixir and 100k DE.

and those upgrades can be specialized on attack speed, poison, damage or whatever. For example a mortar can be specialized on taking more time to throw the ball, and dealing more damage, or specialized on more range, but not both specializations.

more towers couldn't really fit in a map this size, and more attacking troops would slow down some devices so idk about this.