I say have some poped HNM's that you farm pop items off, say level 60-70 mobs.

Also some Roaming HNM.
Have them pop every five minutes like any regular NM, but put a recast timer of like 15min on any member that participated in the kill (like the raids), that way you don't have people hogging the NM.

Though I don't see how any Roaming NM will be that challenging if you can have 40+ people attacking it.
IDK I'm sure they'll figure out a good way to please everyone.