So I kind of feel like the idea behind Naxxramas hard modes and gradually increasing difficulty is good, but badly implemented. There have been gates added to the entrance to each wing, which appear to server little to no purpose beyond prompting you to select a mode upon entry.
How it actually works:
You start in central Naxxramas with the doors to all four wings locked, and must speak to a Scarlet NPC to commit to whatever hard modes you like, once you have committed and locked in, the doors open and you have to wait 15 minutes to make any changes (and need 5 players, an annoying requirement if you want to change things mid wing).
There are four crystals you can activate, indicating each of the four wings. Each crystal activated adds a stacking HP and Damage buff to the entire dungeon (including trash). On top of this, a specific mechanic (similar to BWL trials) is active in the wing selected.
It seems that for each level of hard mode active when you kill a boss, it adds an extra drop to their loot table, as well as extra spliters of Atiesh, a fair deal.
The problem is, you are not obliged to do the wing you have activated, and can simply change it around to avoid the additional mechanics.
It seems like, unless all crystals are off, the gate to the wing should only open if that wing’s crystal is activated.
Example:
My guild feels confident it can do 2 of the 4 hard modes, so it activates the Abomination and Military wing’s crystals, activating a level 2 hard mode. It then proceeds to clear the Spider and Plague wings, with no new added mechanics. Then, we deactivate the first two crystals, and instead activate the Plague and Spider crystals, then head into the Abomination and Military wings, once again with no new mechanics, and completely avoiding the added hard mode mechanics.
How I expected it to work:
When I saw that the wings were behind a closed door, I immediately assumed, that the doors would only unlock if you activated that hard mode. And if you activated none, then they would all open and off you go.
It made sense that you were forced to clear the wing you activated, introducing the hard mode mechanic. Once you cleared the selected wing, you were then free to turn it on and off to increase difficulty of other wings as you see fit. Should you fail to clear a wing with the hard mode on, you cannot then turn it on again.
Example:
Once again, our guild fancies doing 2 of the 4 hard modes, so we activate Spider and Plague crystals, and the doors to those two wings open, whilst the doors to the Abomination and Military are locked. We start in the Spider wing, now having to do the hard mode mechanic, and clear the wing. Now we go into the Plague wing, and are struggling so decide to dial it down.
If we turn the Plague crystal off, only the Spider wing gate remains unlocked, which we have already cleared, and thus cannot proceed. If we instead turned the Spider crystal off, we can now clear the Plague wing with 1 hard mode, but still have to do the hard mode mechanics. If we turn both crystals off, everything unlocks and we can do whichever.
If we then clear the Plague wing with just that crystal on, and want to continue on only 1 hard mode, for the Abomination wing, we have to turn Plague wing off, and turn Abomination wing on in order to unlock that wing. Let’s assume we are unable to kill Thadius, so we have to deactivate the crystal (as well as any other crystals), and proceed on normal 0/4 mode.
Now for whatever reason, we decide we want to do the level 4 hard mode for the remaining Military wing, we activate the Military, Spider and Plague crystals, but are unable to activate the Abomination crystal, because we didn’t clear the Abomination wing with it activated, so the highest we can get at this point is 3 out of 4.
Conclusion/TLDR:
The wings should only unlock if the crystal for said wing is activated, unless no crystals are activated at all (normal mode). Failure to clear each wing (every boss, not just the final wing boss) with it’s relevant crystal active, should mean that it cannot be activated again for other wings.