When the user modifies an asset, such as a texture, the ingame models are updated instantly.
-> awesome live-testing for modders
Far more challenging: make this work for unit stats.
Let modders make a snapshot of their current build via a provided engine tool. The mod can be rolled back to a previous state with this feature. Snapshots could be incremental (only changes to previous builds are saved) or fully-featured (the snapshot itself works like a standalone build).
Incremental snapshots could make use of a version control system like git or SVN, although this would be limited to non-binary files only. It would require a git wrapper in the editor and the modding tools.
Snapshots could contain information that are unnecessary for normal builds such as debug information or generated data from test cases.