One thing that I've been working on is making sure enemies from different levels are compatible with each other. I programmed the enemies only taking into account the other enemies of that level. So, for instance, the UFROs wouldn't kill the Blorps while spinning on the ground. These sorts of things need to be fixed in order for custom levels to work properly.
And ofcourse, I need to get level loading working. Actually, level's can currently be loaded into the editor just fine, but there needs to be a list of saved levels generated so the user can pick which one to load. Well, that's up next.