Summary: Running season-based events has proven successful in other games (e.g. Diablo 2/3, PoE) as a way of bringing players back on a regular basis and slowing decline over the long-term, even when the gameplay hasn't changed. It's something I suspect might help keep WvW alive.
In light of the news that there's still no ETA on world restructuring or the changes that will be needed afterwards, it's looking increasingly unlikely that WvW will have many players left by the time its core issues can be fixed.
This is, obviously, a bit of a problem, and it looks like it could go one of two ways. Either WvW will die, and maybe these changes resurrect it if they're completed (and work), or a band-aid approach could be used to give players some way to find more sustainable enjoyment in a game mode that's not offering much in the meanwhile. I think holding regular seasons could achieve this.
Because the achievement system required already exists, I suspect it would be easy to implement. Once that's done, it requires few resources to spin up each new season. It's possible to include extra changes to make each season different, but this is optional.
For players, it's the regularity and cyclical nature that appears to do the magic in other games where this is used:
There are a couple downsides:
(My apologies if this has already been suggested somewhere... I've done what any unhelpful numpty would do and posted without searching.)