I've been thinking about this as an idea - instead of making a new editor for Loap, making a fork of L3DEdit that edits Loap levels rather than DOS L3D ones.
Many of the things Loap would need, would be fairly trivial to add to L3DEdit. (Many. Not all.)
Downside is this would mean having only an isometric view while level editing. There are a few things that would be hard to visualize in this view, in particular default camera angles. There's also the issue of what happens if Loap in the future supports 3D models for interactive / decorative objects; supporting these in L3DEdit would not be easy.
A bigger downside also is that L3DEdit is not as easily cross-platform. Although it can be (and has been) built for non-Windows OSes, it doesn't perform nearly as well on these platforms, due to using Lazarus rather than Delphi to compile on them. (The worse performance also occurs in Lazarus-built Windows builds, but on Windows the option of using Delphi is there.) I believe L3DEdit runs fine under WINE so this isn't too bad for Linux, but it will leave Mac out.
One reason for this is effort - there hasn't been much interest in making L3D custom content so far, so I'm not sure if it's worth the effort of a full new editor for Loap. Using L3DEdit as a base removes a lot of the required effort.