I think the best cutie mark would be if all 3 (or if 3 other from Manehattan then all 6) would intertwine with one another.
Like each cutie mark would have something to do with one another, at least then the CMC 3 (or 6) would then have a permanent placement in finding or helping others to find there special power/skill/hobby.
The Mane 6 are for Friendship, so why not the CMC 6 be for cutie marks? Heck for the sake of (yes going there) Twilight Sparkle going all Alicorn (glad they did this for the Tirek battle) Then the CMC maybe could be the ones to help all Pony's unlock there magic or maybe (also going there for the shock factor)...
A pony could have more then one cutie mark, or maybe change there cutie mark for a talent or skill that is more special?
The possibilities are endless