No comment on that from me...
But anyway: The idea of an end-of-campaign BBEG is an odd one for me. My games have all been serial but non-linear, and thus the PCs have various foes at various times; moreover, they usually (sometimes inadvertently) choose their own enemies. Thus, it's much more likely that the "BBEG" in a low-level game may be the leader of a bandit gang, the chieftain of a tribe of orcish marauders, or the head of the local thieves' guild than any sort of epic-level or 20th-level being.
As far as my world at large goes (the campaign is centered in Toril, the world of the Forgotten Realms setting), the scale is pretty much like this:
1) Ordinary beings: These are creatures with NPC classes, the overwhelming majority of beings in the game world, from paupers to soldiers to thieves to kings. Even the majority of "mages" are adepts, not PC-classed spellcasters. I use Iron Heroes NPC classes, which are quite weak compared to PC classes, so the differences are VERY apparent. A 4th or 5th-level IH character can probably defeat a 10th-level warrior with ease in combat, and it just goes up from there.
2) Dangerous or puissant beings: This includes the lower-CR type of monster (owlbears, ogres, the standard run of oozes and aberrations) and low-level PC-classed beings. These are likely to draw stares in small towns and villages, and inspire caution even in tough guardsmen and frontier types.
3) Heroic beings: This includes mid-level PC classed types, lesser fiends and celestials, and monsters such as giants, greater hydras, chimerae, and greater aberrations (illithids, beholders, aboleths). These beings are either (if monstrous) dread terrors whose nature is whispered by the most inquisitive sages, or heroes whose fame is sung across the land. The more sophisticated towns and cities (Waterdeep, Silverymoon, Berdusk, Zhentil Keep, etc.) have procedures developed to handle such beings if they run amok. This is where the first echelon of background campaign villains would be likely to lie; the illithid who sends charmed minions and doppelgangers to slay and infiltrate among the PCs, the Zhentarim warlord whose troops turn up as a constant thorn in their side, the dreaded sorcerer who toys with powers beyond his control.
4) Legendary beings. This includes the highest-level (15th-20th) PC-classed types, the moe powerful fiends and celestials, dragons, and the most powerful of other creatures. These beings are famed in song and legend across the Realms, and form the highest echelons of power in most places (the ranks of the Zhentarim and the Harpers, the governments of the Realms' major cities and kingdoms, etc.) These types are likely to turn up as villains only after extended play, and then when the PCs are already in the middle levels themselves; gaining Naergoth Bladelord, Sememmon, or the balor Errtu as a foe takes a lot of accomplishment!
5) Epic beings. This includes beings of 21st-30th level. Such are the Chosen of Mystra, the Inner Circle of the Zhentarim, the greatest of dragons, the phaerimm, liches, the Zulkirs of Thay, the archwizards of Halruaa, and the greatest elven high mages, as well as the Dukes of Hell and lesser demon lords and daemon masters. These beings are either immortal or extremely long-lived, and the PCs may only ascend to their ranks by virtue of the fact that they are destined for greatness. (Moreover, some sort of divine blessing, quest, or other means of ascension is often required.) In order for the PCs to earn a foe of this stripe, they are either (a) epic beings themselves (only one PC group of mine has hit this level, and they're effectively semiretired); (b) acting on behalf of one of the epic beings listed above; or (c) they have something the BBEG really wants (an artifact, the secret to an epic spell, a vital piece of information, or the potential for an archfiend to gain a serious advantage for itself or its masters).
6) Lesser cosmic beings. This includes beings of 31st-40th level. Such are the absolute mightiest of the Realms (Larloch is the only one known, really) and the lesser powers of the Realms Beyond: Archdevils, demon princes, demigods, and some REALLY serious monsters (greater abominations, supreme Far Realm entities, etc.) Even the most powerful mortals are likely to be quite beneath the attention of such beings; one would either have to be doing something that directly serves the interest of a power of equal stature, or possibly be verging on a similar status (although I can't imagine a campaign that gets past 30th level!) to acquire such a being as a foe. Moreover, the vast majority of such beings are prevented from acting directly in the Material Plane, so PCs will interact with their agents for the most part.
7) Greater cosmic beings. This includes beings above 40th level, including but not limited to true deities, archfiends of the highest rank, Elder Evils, and similar beings (cosmic monsters locked outside existence for all eternity, etc.). PCs may fight the worshipers or proxies of such an entity, but they will almost certainly never face the entity itself. My epic-level PCs had a feud against (the greater deity) Cyric going for many years (of game time and real time), but that feud was carried out against his church, summoned fiends, and once a minor aspect of the deity. It is possible that PCs would find themselves made proxies of such a power or sent out on a quest on behalf of the power, but that's about it.
8) Supreme/transcendent cosmic entities. This includes beings of sufficient power to overmaster or challenge even the gods themselves. Such beings would be 70th+ level (since my greater deities top out around CR 67) and able to resist even divine and cosmic abilities with some ease. While such a being could be a "BBEG" of sorts in the campaign, the real fight would be against cultists or ambitious would-be lieutenants seeking to free such a being or bring it into this cosmos.
