If you're an admin, it pays to know your member base very well, which includes knowing if your most invested writers are susceptible to burnout. Of course, it is difficult to identify the rhythms of your players until you have a steady cast and have created a myriad of events and activities for them to try. In the end, you're creating events, rewards, and systems for your site's invested players. Not another site's after all.

Designing events and rewards that allow players to feel that completion is possible, to feel success (on top of acting as narrative tool builders ICly and OOCly) is paramount for motivation and healthy involvement in the hobby—and in your site. Otherwise, events from RP Bingo to the classic mega thread can become daunting work that may not be overcome on the part of the player and occasionally, on the part of the staff. We are all susceptible to that "RP as work" mindset, especially if we consider how we may view our "posting queue or tags". In an ideal world everyone would perform healthy approaches to our favorite pastimes independently, but that's not easy! It does help when particular systems or guidelines are in place to encourage more sustainable practices.

I think it's quite tricky to balance an event (should there be rewards apart from plot hooks, narrative connectivity, the satisfaction of character development, etc.) that honors the most active of players and acknowledges those who play casually in a satisfying manner. To help circumvent this, I believe it's better to have various kinds of events throughout the year. Some that are geared toward and reward the most active concretely. Some that are geared toward more light engagement with a leveled playing field. Some that spotlight particular genres, particular processes of writing, etc. Perhaps, ensuring that your "rewards" (if they are physical) are not forever gated and are able to be accessed at a later date, can help mitigate any FOMO. However, I do believe that the most active and invested of players will ultimately be (and should be) the most rewarded in the end. You give as much as you get!

Other aspects that need to be considered are site economy, staff workload, other systems or mechanics that interface with the event/the site as a whole, logistics behind running an event, and what an event is aiming to achieve tonally, narratively, etc. 

The aspect of player burnout, rewards, and activity is a tricky and complex issue, I think! At least, that's how I feel as someone running a site. In the end, I think trying to derive enjoyment and satisfaction out of the stories we actively weave with our characters and treating any fancy title, item sprite or forum-dollar as an optional cherry on top is the most sustainable way to go.