When in Model Builder, it would be really useful if Sub Models were highlighted as being submodels and not just another step/tool in the chain. When my colleagues are editing/troubleshooting models that I have created, it's not always inherently obvious to them that there are submodels present unless I do something like the below. A simple highlighted border around the yellow tool symbol, or asterix against the name with a label. Some idea examples below.
Current default display:
Submodel with border:
Submodel with Asterix and associated label: