I'm not sure there should be an "epic" tier in the sense of a bunch of levels that are designated for doing epic things. Several people have said they want epic to change the feel of the game, but I'd bet there are plenty of people who just want the numbers to go up, or are at least indifferent to that change in feel.
After some thought I've had a crazy idea. What if, just as characters choose a class, background, theme, etc., campaigns (optionally) did so as well? Let's call them campaign themes for the moment.
What if the party as a whole "leveled up" in ways determined by the campaign's actual events, but mediated by a campaign theme? The party as an entity could have resources, and the kinds of resources that are made available could depend on the campaign's theme. A campaign where the PCs aspire only to become rich would develop in a much different direction than one where they aspire to be heads of nations, or slayers of gods. In this case, running an epic game is a matter of selecting a campaign theme so that the party naturally develops epic-feeling interactions.
Although most aspects of a campaign theme would probably not affect individual character advancement, I think there is room for this as well. For example, if a campaign is forming a kingdom from the wild, it is often the case that one of the PCs will be king. Well, maybe this campaign theme offers kingly character abilities that the king could take to support this kind of play. This probably isn't raw personal power, but in a kingdom game it may let him call upon his strategic resources (like an order of knights) in a way that affects a tactical situation more powerfully than usual. Likewise, a character playing a general in a campaign with mass battles could gain character abilities to interact with the mass battle mechanics. But if one of the PCs is the king's loyal servant and protector, he could still choose to concentrate only on tactical combat. The key is that the campaign theme should give ways for everyone to contribute to the larger picture, but let characters embrace it to different degrees. It should also be "balanced", in the sense that any changes in power will tend to affect the whole party moreso than any single member.
OK, so how does this affect the introduction of "epic" play? I think the best thing might be to introduce campaign themes in the core books, detailed mostly in the DMG. The themes in the core books would be tightly focused on dungeon crawling, urban intrigue, and similar things that don't demand introducing huge amounts of supporting material but still speak to D&D's traditions. I might dedicate a chapter to them, with 3 or 4 light themes. The first goal is to make sure they are mechanically integrated with the core game, so if the PHB contains 20 levels then both dungeon crawling and kingdom building campaigns run from 1-20. The second goal is to make sure the concept gains mindshare from players and DMs.
After that, one could introduce new campaign themes in lots of places, like additional DMGs. Campaign settings and APs would be ideal. It's practically an invitation to reintroduce Birthright without making any changes to the core game. Heck, even a new Stronghold Builder's Guide would have a place, for those who want to gain and run a keep. I think there could be room for a dedicated book of Epic Campaigns with all the mass battles, god slayings, and kingdom-runnings one could want. And unlike a traditional epic level handbook this is one a DM might be able to use from level 1. That along could raise interest in it beyond what an ELH normally gets.
The idea of campaign themes has some obvious weaknesses, but I'm not sure if they're deadly. The biggest one is probably that campaigns often develop organically, so a theme isn't obvious. Second, themes can and do change. Third, a theme shouldn't *prevent* a game from changing directions. Finally, all of this requires some serious player buy-in.
Assuming campaign themes are used, I think the main thing is to make sure they are fluid. That is, they mesh well from one to another. Perhaps they can be retrained, or every "party level" is associated with some theme, but these change with the story like 3e multiclassing. For example, an epic campaign theme isn't necessarily appropriate until the characters control some land. In that case, in a zero-to-hero setting the first campaign theme might be acquiring a keep, at which point things smoothly switch to more epic concerns. In a different game one or more PCs might be aristocratic and have this from first level already. In other words, themes change when goals or story conditions are met that somehow change the stake or scales of the game. This could work in cool directions: perhaps a campaign starts with the prince inheriting his kingdom, but in the course of play he and his court are deposed. At this point the kingdom-running theme falls to the side, but he goes adventuring with his most loyal companions for the relic which could return him to power. While in possession of the relic it makes them aware of a cosmic threat, so when he is restored to his throne they turn their attention to this new threat.
If something like campaign themes were implemented, it's clear to me that the worst case scenario would be something like 4e skill challenges. That is, the system doesn't quite cohere but the rest of the game can ignore it just fine. As with most failed systems, DMs will just grab the elements they like and keep on trucking.
Alright, I've got to stop writing. I'm interested to hear what you think. Cheers.