Hi guys, first of all, this is my first message, so hello and nice to meet you all.
I know I'm late in this discussion but I'd love to have an additional feature in the format, sorry if this has been covered before (I made a search, but didn't find anything).
Having a custom property, or even better: an array of custom properties. Just with two fields, 'name' and 'value'. In my current game, where I implemented my own skeleton based animation system, some bones were "attached" to physical objects, and some other bones weren't, so I'm able to switch from an animated character to a ragdoll quickly (just create a physical 'bone' attached to its parent and let the physics engine do the rest). So in this case for example, if I want to port my poorly programmed animation system to Spriter, a custom property for each entity/bone like "isInRagdoll" would be quite useful.
Just my 2c :)