A mode to allow messing around with fighting enemies and making builds.
While I feel like part of the game is trying out builds in stress inducing situations as you try to survive while also exploring builds. However, at some point I just want to be able to see what kind of broken builds one could make even though I may have trouble getting said build off of the ground. Thus I think after a certain rank (a pretty high rank; after unlocking all the mods) it would be nice to have a sandbox or at least some way of exploring how mods work together. It'd be important to design it such that its still way more rewarding to get the build running in the actual game.