Yeah, these are just suggestions offered humbly - it's your vision.
I would prefer more fundamental game systems creating asymmetry over Armor/weapon types: pikemen having long attack which prevents cavalry charge on hit and knocking back for example.
Special abilities are the strongest candidate I think, turning battles on good usage and counter-usage. The game already requires constant attention for placing units, so this would not change the game flow for the players. I love the idea of Elves casting an aoe root which delays part of enemy army. Controllable abilities would also work really well with terrain features for changing battle flow.
Pacing: I tested a little more and the pacing is quite good gold-wise, I would maybe lower it -20%. I also recommend not having several settings for the configs, easier to tune a single config and that keeps the game experience and balance same for every player as "Intended Experience".
One further thing I would like to add is that I think the ground textures require a rework as priority one. They're visually cluttery, out of scale and poor resolution. The aesthetic experience of the game will greatly benefit from cleaner and simpler ground.
- I would make the first wave spawn in 60 sec so theres time to build and not instantly lose the game in first wave