I was thinking recently that for edittime properties there is 2 types missing that i would find quite useful:
The first thing is paired/grouped values like you already have with the Common properties for Position and Size. It would be nice to have these available to use as a with regular plugin and behavior properties.
Then the other thing would be a separator line. Each item is already separated by a thin line so maybe it could just make that line chosen color and also slightly thicker, maybe with a optional title text. This would be helpful if a plugin or behavior was slightly more advanced and you wanted to organize the properties into a few groups.
I think these would be quite useful as new ept_ types, hopefully you will like these suggestions also. Many thanks :)