Popping in with my tarnished two cents here, I'd just like to make a note of my immediate interest at the idea of 'epic AMPC's.' Traditionally, this role would be something we'd normally see belonging to the DM's, with creating extremely powerful foes, or using ones available from the many big nasties that Ravenloft has to offer. However, they 'are' a lot of work, and mucking about with story centric enemies isn't something that is done easily or lightly, hence we've seen a dearth of activity on them (at least outside of NCE's) for quite a few years now.
We have however seen some of our more trusted community members create fantastic stories and experiences with stronger than average AMPC's, with higher than usual levels. I won't call out names because its unbecoming, but in the instances I know of it happening I wholeheartedly approved of it. It worked. It created some significant challenges and strong storylines, and not least because these were and are great players.
There are certainly 'tiers' of AMPC. Not simply in what the template offers, but also what levels it might have, which in turn can dictate how strong it actually is with all the spells and such available. Those 'tiers' as they were can certainly create some very interesting possibilities, but a lot of it comes down to what is frankly already in place - the discretion of the CC and DM team to approve it, and that is based upon the judgement they have of the player involved. Some of us aren't quite there for this extra power for the creation of story - but precedent shows we 'do' have exceptional and trustworthy individuals who are rightly given a bit of extra kick, and its paid off time and again.
At present, if you were to look at the power (and the correlating level of responsiblity of use to go with it I would rank it something vaguely like this - keeping in mind NCE AMPC's start at lvl 7, and outside of that they start at lvl 13.
Tier 1 - NCE AMPCs,
Tier 2 -Ghoul, Wight, Wererat, Werewolf
Tier 3 - Mummy, lvl 15 tier 2 AMPC, vampire
Tier 4 - Revenant, Wereraven, lvl 16 - 18 tier 2/3 AMPC
Tier 5 - lvl 20 AMPC, Theoretical Epic Style AMPC?
The rarity you speak of before is implicit in this sort of vague (and not at all set and defined, emphasis on vague) tier levels. Tiers 1 is pretty easy work, because the NCE window limits them. Tier 2 is the basic standard, tier 3 has more responsibility because of the power in the character. Tier 4 however, is being much more strict. Revenant is a very difficult AMPC to get because it is 'very' strong now, and also has a level of roleplay implicit in it that is somewhat more developed. Tier 5, the last, is the strong stuff. Its very limited, likely to be declined, and probably going to need some ongoing DM support, so its more often than not going to be declined if things just aren't quite feasible or there are legitimate concerns had. But there 'are' players amongst us who can be trusted with that level of responsiblity, and we've seen that before. If the epic AMPC's in question - Death Knights, Liches, Fiends, Arak Sith and other creatures besides being possibilities - were placed at such a level of responsibility, sure - people may yearn for such tasty things. But we can by means of our team be sure of a couple of things - first, that it shall go to players who can be trusted with it, second, it will give players adversaries that honestly they absolutely need to have and quite frankly crave for, and third, it can even give people an example to strive towards in their playing - the knowledge that if they reach a standard of playing with their fellow players, they can experience something just shy of being a DM themselves.
There are so, so many great and responsible players on the server, who could both make great use of and deserve something like this. I know its just a throwaway sort of comment that gave rise to it, but nevertheless, it certainly did pique my interest and so here I am giving my thoughts.
Personally I'd -love- to see Death Knight. Even if it was but one, one is all you really need.
EDIT: Oh, and for other tier 5 style AMPC's - I'd personally go with Lich, Ghost, Malignant Werecreature (Cursed) and Fiend. Each of those are strong as heck, requiring some fairly specific things to kill properly in some cases.