I am interested in seeing more factors tied to the account rather than the character and in the form of unlocks rather than items.
Most MMO elements are tied to the character. Your level, skills, reputation, achievements: all of these are character-specific. You may be able to trade money and equipment between characters. Some games are progressive enough to let you share a few items like a friends list, chat channel, guild affiliation, or key bindings across characters.
Some of my interest comes from being an altoholic. If I have a dozen characters, a bonus that applies to all of them is more interesting than a single-character upgrade. It is secondarily of use to the hardcore with multiple level-capped characters, less so to players who devote themselves to a single character. It makes it a lower-investment decision to try new character options, and it retains the illusion of progress and permanency rather than making each character feel like something entirely new. It may also encourage thinking of “avatars” rather than separate characters, and encouraging altoholism may not be what a developer wants if characters take up a lot of storage space. Or it may be exactly what a developer wants if it keeps people happily re-playing the low levels on a dozen characters, continuing to pay for the same content.
Moving to a single-character model will have some related effects. One can capture many of the benefits of both “single character” and “many alts” by increasing the flexibility of one character to change roles and careers. One might also give all characters on an account pooled experience, as Alien Swarm does; the player levels, not the characters.
Having unlocks instead of items works on a similar principle of increasing flexibility per character. City of Heroes does this well with costume pieces: some of the most sought-after rewards are new cosmetics, and instead of giving you a cloak or something, the game permanently unlocks that option at a tailor. Many games act similarly with unlocked abilities or talents you can swap between. Team Fortress 2 straddles the line here with item drops that are effectively unlocks because you have more storage space than you need.
Existing games could patch something like this in by introducing vendors that will sell you items (cosmetic or not) that you have previously acquired. That would require adding a table per character or something, potentially a large one, but it would give you a way to retain the reward as something you always have access to, rather than a temporary item. See Warhammer Online’s Tome of Knowledge as one way you could track which set pieces you have acquired before. (You could combine cosmetics and sets by having completing a set unlock that as a cosmetic option: click the button on your achievement screen, and your character appears in that full armor set.) It might apply only to soulbound items, we need to keep people from buying all the possible rewards from a quest instead of choosing one, insert your details to be worked out here.
Also under details, see re-balancing all those grinds on a per-account rather than per-character basis. Kill how many rats? Also see also the scaling-related trade-offs that might encourage one high-level character (like how most in-game economies make low-level cash rewards meaningless in the player economy) or might encourage degenerately many low-level characters (say, “have 100 levels of characters,” which is most easily reached by earning 1000xp on a lot of throwaway alts).
Some of my interest is the illusion of permanency. We know our “persistent worlds” persist only so long as servers are up. While they are up, let us indulge our magpie-like urges. I will almost always prefer working towards a permanent upgrade rather than a temporary one, even if I will never actually use the permanent one. I recognize the perverse stupidity of capping LotRO virtues that I do not use, but it is a permanent character improvement, whereas I know I will be recycling my “legendary” equipment frequently. If I ever want that virtue, it will already be capped and waiting. Similarly, if I can get a permanent account upgrade, even a trivial one for a role I do not play often, I want that. It feels like an investment.
That seems like a feeling developers would want to encourage. Committed players have higher retention. It is related to those veteran or elder or whatever rewards that encourage people to stay subscribed for a long time. It is, however, a retention scheme, not one intended to gain new players; you could potentially alienate new players by having long, account-based upgrade grinds that provide no benefits to newbies. It would be that EVE effect where someone feels like he can never catch up.