I went through the thread to determine exactly which of my levels I had already posted here, and then went through the ones I hadn't and polished them off. That process also gave me one or two additional ideas. This is the result: four levels for SubTerra 2, and four for SubTerra.
SubTerra levels
Clone Maze: I already posted a draft of this, but I finally managed to make it work by reducing the number of clone machines and adding some one-way blocks so that most of the cloners can't be activated more than once.
Friendly Fire: The idea here is to get the skulls to destroy themselves. Or something like that.
The Ultimate Dungeon?: Or not. This is another chain of rooms type of level; you go through one room at a time until you reach the end, and there's a save point between rooms. It's not particularly difficult, though the key puzzle may take some figuring. (Unless you watch the solution.
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Save the Children: Something I thought of while making Cloning Madness II. In Cloning Madness, I used cloning machines to generate players; in Cloning Madness II, I used portals and sorcerers. That reminded me of Skelwings and their ability to generate anything at all, so I wrote this level as one where skelwings generate players. It's technically a two-player level, but player one does most of the work.
SubTerra 2 Levels
Cloning Madness II: Basically the same as my Cloning Madness for SubTerra, but re-imagined for SubTerra 2. Players are cloned both through sorcerers and through portals.
Complex: A level using the Mountain King as a sort of remote drone. It uses two players, though don't play equal parts in the solution (at least the hard mode one).
The Vault of Eternity: I think I posted this one before, but there was an issue with the hard mode solution (it was impossible because of an extra player appearing). This is a fixed version which is beatable on hard mode.
Open the Vault: A puzzle with linked boxes. Not terribly hard.