Nightmare mode should be tuned such that it absolutely requires every raider to put in the maximum possible effort in terms of pre-raid preparation. It should also require an exceptionally high level of execution and strategy to complete encounters at this level. There is absolutely no reason why nightmare mode content should not be tuned to this level given that normal mode gives the more casual player an entry point to raiding and hard mode gives them something to aspire to.
Given that Nightmare currently gives out almost completely identical gear as Hardmode.. it should not be tuned as you're suggesting yet.
Ideally, Nightmare should've remained locked until the reward it gives would fall inline with the difficulty that you're suggesting in terms of tuning. If they tune Nighmare to the extent that you're suggesting now while still dropping the same gear that's available in Hardmode... there would be very little reason to run it. Granted, this problem is still evident in it's current iteration, but the rewards fall in line with difficulty difference.
Nightmare rewards only slightly better than Hardmode for a Difficulty slightly harder than Hardmode. This is a balanced design. If/When Nightmare is tuned to a much more difficult experience, the rewards will need to be significantly better than Hardmode rewards.