They've already got every single bit of code that I'm talking about already in game except for maybe the pet targeting - that's the only "If" part.
Zonewide auras exist in at least a dozen zones that I'm aware (probably well over that now), reducing HTH damage effects exist, reducing spell damage effects exist, reducing overall damage exists.
Adding absolutely nothing new besides an NPC in each zone to do the AE is the entirety besides using effects they already use. If any of that stuff would be breaking stuff, it would be doing it all over the place already. Heck, Prathun (I think it was him at least) over TLP2.0's buildup confirmed it would be "easily doable" - and you know how much of a pessimist he is - he just didn't like the idea of fielding the "Why does x work differently than Live" questions all the time or "Why does my spell hit less than it's written?" in the case of the casting debuff.
Period. That's the problem is confusion over the different results of abilities and such. The difficulty should be moot unless whatever dev it was back then was full of shit, which I somehow doubt things have gotten harder in 5 yrs.