A lot of new mob attributes can already be changed since v2.7.0, but there are a few things remaining that might be cool as well. This ticket keeps track of those ideas:

• Changing equipment (armor, held items). Maybe by having slots which players can place items in?
• Option to change the size of mobs.
• Option to have riding mobs.
• Possibility to change the mob type (shop object type) in the gui.

To keep the editor organized (once there are also more other editor options), maybe a separate gui for those appearance related options might make sense.

v2.22.2 will add an equipment editor for mobs. The block carried by endermans will also be editable by this.