Here are just some ideas that I would like to propose.
I don't doubt most of these options would be pretty easy to implement.
1. Map preview window.
When selecting a map to play it would be nice to see a little preview of the map to remind the player which level it is.
2. Freeplay mode.
No leaderboard rankings, select the level, ton of cash, play with traps, layouts, whatever not having to worry about coins.
Play either story or endless.
Again, not ranked--just around to have fun (it's a game afterall).
3. Replay files would be nice, not necessary--but nice.
4. A small array of icons showing the loadout of your coop buddy would be nice and how much cash they have. Maybe as tooltip in the UI.
5. Randomized loadout option. Picking 1-3 weapons, 1+ trap, and 1-3 trinkets from your inventory before starting a map. (roll your own challenge levels).
6. Mod support for customizing wave (with no leaderboard rankings).
Change the map's name, limit loadout size, set wave values without actually tampering with the OMD files.
Like how doom used to do it by placing PWADs into a subfolder and using -wad or whatnot.