They could just do it how GW2 does it:
- Have world bosses spawn once approximately every 2 or 3 hours, so that people have a decent chance to encounter them regardless of when they can play. Make it predictable enough so that timers such as these can be made for each server.
- Players can participate as many times as they want, but can only receive the rewards (i.e., Odin's Mantles, Behemoth Horns, etc.) once per day to prevent them from being too common. Players will then be more motivated to try and get gold so that they earn more horns/mantles for that day.
Other things they could do:
- Bronze rating should be 1 item; silver could be 3; gold can remain 5. It's frustrating to show up late to these events and end up not getting anything even though you put in a decent amount of DPS, healing, etc. This way, in conjunction with the "one reward per day," would reward doing a good job, while also rewarding general participation.
- Release a multi-part version of the Herklaedi armor (body, arms, legs, and feet pieces) for glamour purposes, or other new items so that players still have a motivation to participate in these events.
- Make the items also tradeable for things such as tomestones or GC seals, in case you have all the rewards already or simply aren't interested in them.
- Introduce new world bosses, and/or have other "lesser" world bosses (Lazy Lawrence, Gorgimera, Cancer, etc.) drop token items instead of minions. Have new rewards you can get from trading in items dropped from their respective bosses (minions could be one of these).
- If you REALLY want to encourage participation, have certain new gear drop from these bosses (such as ilvl110 Odin armor), but have the drop rate extremely low, sorta like how Atmas are now. Trade-in rewards could also be included so that you don't have to spend your tokens if you're lucky.