I've looked around and it doesn't seem like theres a whole lot of support for custom level designs, or any mods that make custom levels, or anyone discussing the idea (that I've seen, I'm not in the discord).  Would a level editor be possible in a distant future update? Is there a way for modders to make levels themselves right now? If so, how? 

the dev has shown interest in adding in a custom level editor in the future, though it's low priority at the moment since it would take a bunch of effort to add in

as for modding it in, it wouldn't be very easy to do since the game relies on unity scenes for the levels (so you can't do much atm)