Thanks for typing all this up, great feedback here!
Traps are an interesting idea, how are you thinking they'd be distinct from towers? Would they get used up as a minions enters (or multiple of there's >1 activations), then you get your coin back? I can see that being useful when/if I add routing to towers, so you can create trap mazes.
Great point about the screen edge spawns causing problems with towers. I'll consider having them spawn from the map edge, but I'm worried it would create situations where the action is too far away from the player (especially if they get stuck behind obstacles).
Flying and ghostly enemies are definitely planned, both able to pass through obstacles!
The accuracy upgrade increases the arrow's travel speed, yes, not the player's walk speed. I find it useful for defending the portal.
Having ice slow down attack too is a great idea! I personally do feel it's useful, because it can slow down an unlimited number coming at you, unlike acid which can only kill a few. It however is less useful that fire because that's your real crowd control.
That said, I'll sadly be nerfing the fire effect. I was actually looking into some performance issues and realized it was mostly due to a runaway effect in fire spreading. Basically when a minion is in the process of dying, but not dead yet, fire could spread back and forth multiple times. This basically meant a minion who dies while on fire turned into a bomb that spawned lots of fires which spread to adjacent minions. It's kind of cool actually, and I'll almost certainly be adding bomb effects like this later, but it was actually a bug that was allowing fire to spread and wipe out whole crowds instantly.