I really liked @tragedyjones's idea on Reno for 'Seasons' -- a staff-run broad-ranging plot that was planned to run around six months. While they tended to be sphere-based, they weren't other-sphere-exclusionary, which was useful; I could easily see the concept being opened up to broader plots.
The short term factor is helpful in that it isn't the "whole story" of the game: it's a big story in the game, but it isn't the only one and there are room for others thereafter. That means people who don't or can't get involved with, say, Season 1, can hop in on Season 2, etc. Edit: Also, and more importantly? Once "The Big Plot" is over, it can feel like the game has come to an end. If that's the design and intention, cool -- but if not, it can leave people floundering.
One of the things that has been off-putting to me about metaplots is the idea that it is the core story of the game: singular, in many cases. Realistically, I think a game (and its potential for longevity) are served better by something like the above: shorter-term, broad-ranging plots that can run their course with another picking up thereafter.
The other big bonus with this approach is that you can have multiple people running them, for instance: Season 1 is coordinated and headed up/run by Storyteller A and their team, while Storyteller B and their team work something up for Season 2 that has a different feel and focus and will have the potential to draw in a different core of players. This way, even if you have a few 'plot hogs', one story is set to start at the end of the last or with some overlap in one direction or the other. That gives the Season 2 interest group, if they're following the few teaser plot threads thrown out that aren't part of Season 1's plot, a potential jump on involvement while the 'stars' of Season 1 are still busy being spotlight hogs. (Some people do this on purpose, some don't, it's hard to avoid either way, and staggering stuff a bit like this can help avoid them always being at the center of absolutely everything.)