Game Design
Level Design
Game Theory
Misc Art Button
3D Models

Acceleration Catacomb
Race Track

I have found no better place (short of a professional test center) than matches with friends for picking apart your level designs. Watch new players hard. Study fun and failure to help improve iterations. Halo Forge saves all match data so you can rerun entire matches from any player's viewpoint and view heatmaps of matches on a website. It requires more patience than a PC editor to build with a controller but recording tools like this easily make up for that.