Not necessarily - you should be able to get a variety of implements by different proficiency bonus and properties alone - no need to touch the damage dice. For example, in your list, the rod could have "high crit" as property, the orb could have "brutal 1" and so on (of course, you would need to take this into account for the power design, but that was done for the weapon powers already).
However, such a model has a "flaw" - implements are class-specific, weapons are freely accessible (with feats), which means that each class would need its own implement set to give choice.
Cheers, LT.