Firstly, Hi! 
Purchased this game not too long ago, and have been playing it non-stop ever since. Have read many threads on this forum. Fantastic stuff.
My suggestion is the ability to issue private challenges with a hidden deployment. (Like Expert mode in-game).
I’m not sure if its a valid idea for the public challenge list, but between friends would certainly make it much more interesting. Certainly removes the heavy advantage the challengee usually has.
My understanding of game development would lead me to think this probably wouldn’t be extremely hard to implement either.
Thanks for the consideration.