I know replacing models and such is quite easy but has anyone looked into making custom levels yet? I'd love to make hacks for this game.

I managed to get the level open in TrenchBroom!

I am very much interested in making a level for this game, seems like it would be a fun level-design challenge.

I'm going to release a quick video on the process on my youtube channel soon. I've already made a quick proof of concept level.

I just released my video on the topic of custom level modding