Currently the material tool will override all materials on a model, I request the ability to replace specific materials on multi-material models. This would allow greater flexibility in model manipulation.
For instance, say I create a model with four materials, three of which are used for the model itself, and one is used as a decal. Currently, there is no way to change that decal without giving that model multiple skins, or creating one very large UVMap texture for every possible decal, which would be very size-inefficient. If you were able to change one specific material, you could change the decal without altering the other three materials of the model.
Another idea is altering the color of a specific material on a multi-material model.
For example: Making one model for a team game, and then changing the color of one of the materials based on the team color. Much easier than making multiple skins for that model for every possible team color, and forcing you to add additional skins and recompile all models if you wish to change or add teams.
It seems like both would be possible to do, and could expand gamemode possibilities greatly.