I'm fond of when a character changes something of their base look/design to call back or honor another character they were close to (taking up their old weapon, wearing a piece of clothing or an accessory that held meaning to them, etc.)

That and when a design change in and of itself reflects character development: like a previously uptight person letting their hair down to symbolize them letting go, or a person who changed due to trauma having a physical mark to go along with their personality change: visual storytelling like that I always get a kick out of.