A useful feature would be the ability to combine multiple models together in order to create one combined model. These secondary models wouldn't need to be complex, and could be limited to adding cubes / texture overlays to the base model.
Possible Usecases:

• Adding cosmetics like hats or other clothing to a model without needing to edit the model
• Making it possible to share addons that could apply to any model rather than just one
• Allowing quick changing of appearance without needing to swap between variants of a model
• Reducing the total number of models required in order to have variations
• Creating models that integrate with existing normal minecraft skins instead of overwriting them

This list could go on just about forever. If this is possible, it would make a great addition. The only way to achieve a similar thing currently is with moreplayermodels, but that doesn't even have a tenth of the features that cpm has.