(Would be nice for C2 but will probably have to wait for C3 if the idea is supported.)
The ability to tag and/or color instance variables would be very helpful, especially if you've got teammates working on levels that aren't familiar with all of them. Some examples...
Editor: These are variables that can be freely modified in the layout editor, like the contents of a treasure chest or the item a particular enemy instance should drop.
Private: "Internal" variables that are only modified via events and should not be touched in the layout editor.
Family: These variables are inherited by a family and are not unique to this object. (These should be automatically separated from the rest, really)
States: These variables control the object's states or mechanics like Attacking, Climbing, Dashing, etc.
Stats: These variables define the object's stats, like HP, MP, ATK, DEF, etc.
Currently you can add prefixes or whatever to help define the variables, like E_Contents, Private_Timer, Mech_Jumping, etc. but that can get messy and make expressions really long or hard to read.