First off I suggest an enemy that walks in reverse. Like an escaped prisoner, they start at the end of the map and walk in reverse, to the start of the map.
Will have higher speed but low damage and hp. Could be interesting and have alot of variations, elves, trolls, whatever anything could be an escaped prisoner.
It would up the game because your defenses will have to focus on more paths.
Next I was thinking of a mage like enemy that has 2 mindless drones walking infront of him. They got like 100 HP each, one has low magic resistance the other low physical armor, if you kill one of them, they split and become 2. And so on. Unless you kill the mage, which walks a bit behind them and is more like a slow tank unit. Killing him/her will finish of all drones.
A variation of this could be a unit with around 3k HP. if killed splits into 2 with 1500 each, then 750 each for 4, and so on till you got a bunch of low HP weak units that die easily and end it at that.
Then theres the portal unit.
A small troll like fast enemy with high armors, but lower HP. He runs as far as he can before the portal machine arrives at the start of the path.
As soon as it arrives, the troll opens a portal and disapears, then the portal machine starts to spin and opens a portal and all new appearing enemies will teleport to where the troll sacreficed itself.
Destroying the portal machine will break the portal.
I also suggest we have a new use for the extra stars, preferably extra waves or hero/reinforment skins
I think these would make it alot more challanging and fun, idk do what you want with these suggestions.