A: Change item set bonuses to cap at one less than the total set items. In other words, you would get full set bonuses from 'Immortal King' after getting any 5 set items out of the 6. That way, if I preferred having a 2h sword over a mace, it wouldn't cost me what would likely be the best set bonus.
B: Conversely, it would also be alright if the last set bonus for completing the set was one of the less desirable bonuses the set offered. While still a bonus, it would make having the full, complete set more about bragging rights and aesthetics than that last, epic bonus. Those of us choosing to opt out of one or perhaps 2 set items wouldn't be as severely penalized.