I'm interested in discussing this topic with regards to what ESO could reasonably do, but it applies to MMOs in general as well. Just kind of a general game design thing.
The whole point of an MMO is the community. The community is what creates a majority of the content, and as such a strong player-base is important. There always has to be enough people involved in the game to make it feel like a vibrant, living, breathing world. But to do that, players must feel a sense of purpose. If players don't feel engaged enough to log in, that's another piece of the community gone.
This is where the various progression mechanics come in. Whether it be something like leveling up alts, getting end-game gear, the dynamic PvP or whatnot, there always has to be some kind of greater goals for players to work towards. The most straightforward is sheer power, which most RPGs defer to. But such vertical progression comes with great pitfalls. Power creep manifests itself in many ways, such as an even larger grind for alts and new players, unbalanced gameplay, and a severe loss of purpose for lower-level zones. The Champion Point system has been a great method of addressing this, but it too will ultimately lead to power creep.
Then there are things like Housing. Housing systems are extremely open-ended and allow players to sink their time and money into something creative and expressive. This is great for the community on multiple levels. I would also have similar praises of ESO's crafting system. I do hope the two are intertwined.
One idea I've had (which unfortunately won't be a reality) is seasonal progression system. It's a vertical progression system that resets every "season", which can be based on time or major expansions. As an example, imagine if Veteran Ranks were re-purposed for this. At the beginning of a season, you start at v1 and must engage in content over time to reach v10. There's not a huge difference in power between v1 and v10, but it's significant enough to make a difference. You level up over time, get some incredibly strong gear, and complete high-end content.
Then at the end of the season, your rank and gear are squished down a few pegs to make room for the new season. v10 might be knocked down to v5, or something, so high-end players have a rollover advantage, but must continue every season to maintain that advantage.
But I'm interested in hearing other players' ideas on this sort of thing. What systems do you think could extend the game's longevity? What ideas are feasible, and what are unrealistic?