I've spent quite a lot of money on other tools to either have them fall flat or no be exactly what I needed for simplistic modular level design. I had an idea of a tool I wanted to write, but it took a good man named Delamore to motivate and assist me at the earliest stage of development. Because of that running start, I was able to complete work on the ultimate staging tool for my model packs.
Enter MAST, the Modular Asset Staging Tool!
MAST runs in the Unity editor and uses prefabs, instead of raw models. This tool even provides a way to convert all of your existing models to prefabs with the click of a button. It strips out all materials - removing duplicates of the same name, splits the model into meshes by material, then creates beautiful prefabs from these models.
Using MAST is actually fun! Simply click a model in the palette, hover over the grid, rotate or flip as needed, then click to place it! You can even flip or rotate a model after it's been placed There is also a built-in "randomizer" for varying the looks of trees, rocks, and other non-modular models. The randomization settings are extremely customizable in the tool's Settings window.
Once the level is complete, you can strip out any MAST code from the GameObjects. You can even merge your models afterwards for performance. Before merging, move doors and other dynamic objects out from under the parent holding the other MAST models. You can also change the destination parent for all MAST models while placing, so you don't have to move anything afterwards.
I also created my first uvmapped model kit. I created pixel art textures, applied to low-poly models, for a unique and sometimes voxel look. They are included with MAST and I even used MAST to convert all the models to prefabs.
This tool is far from complete, and I have a list of features I still want to implement. I released it now, because it works relatively bug-free and the core of the tool is solid. I created a discussion board to post feature requests and bugs at:
MAST itself can be downloaded at: https://fertile-soil-productions.itch.io/mast
Eventually, MAST will also be available on the Unity Asset Store.