I'd probably end up getting frustrated with this idea in practice cuz I don't do many NiM modes, anyways here goes...
As a way to release new HM tiers along side NiM for previous tier, gearing could be done in a way that the HM drops have a higher rating, but aren't necessarily best in slot. Currently we can buy top rated gear with comms...they often come with lettered items though so they aren't necessarily the best in slot. I could see the new HMs dropping that type of gear, the NiM dropping optimized gear a tier lower and comm gear shifting to lettered versions of NiM stuff.
So, say the next tiers of gear are 81 and 83, the layout would be:
New mythical HM Op - drop set bonus gear with lettered 83s
NiM - drop set bonus unlettered 81s
Comms - buy non-set bonus, lettered 81s
This would require NiM content to get best in slot, but still allow for pretty solid gearing with HMs to help progress on NiM for guilds that aren't on the leading edge. Then, when the NiM of the new mythical HM op drops it would have the unlettered 83s and comms could buy lettered 83s at that point.
Sounds like way too much work and would simpler to release a new raid in 2.9 [Or a new expansion? 3.0?] with higher level gear.