If anything they should add one piece of content with each Coil version that is nothing more than gloating and a achievement reward maybe even a minion or mount where needing an average il of 110-115 is needed for just a chance at beating it.

I think the issue with the strongest loot coming from coil is that once obtained it does nothing more than just make Coil easier since there is no other content which outranks Coil. If you simply add a battlefield with a higher difficulty than Coil it would at least add that little bit of extra challenge that some players may want and just be another thing for players to achieve just for bragging rights.