Ah yes Murderball! I like this game mode but as you alluded to it suffers from the whole team pocket healing one person and turtling.
How would turtling be prevented? How do we keep the crystal moving around? How do we make healers not OP in this game mode? For example, what if you couldn't be healed (like when you are in the Arena sudden death Acid Cloud) while you carried the crystal, good idea? Bad idea?
If you couldn't get healed with the crystal you would just get focused and die in < 5 seconds.