There’s a few things that I think are missing:
- participants on the calendar level, not just the calendar event level. e.g. invite participants to the Nostrasia calendar, but not necessarily need to invite everyone to each conference session.
- not sure if having participant lists on the calendar or calendar event itself is the best place for it at scale. I’m worried about excessively large participant lists bogging down the main purpose of the kind - to describe the actual calendar event, not necessarily who is attending. Think about a 1000 person conference like Nostrasia.
- allow multiple locations instead of just one. In the office setting, you can have the meeting in multiple meeting rooms in different offices. At Nostrasia, the sessions were live in Tokyo but streaming in the Hong Kong venue.
- the top-level “content” field should not be optional (but it can be empty). NIP-01 appears to require it.

Admittedly, I have some tunnel vision as I haven’t actually put it into any user-facing client, just the iOS SDK. I did ask @zach the same question for @Flockstr and he said the NIP worked well for him. 

 Participant lists on an event can become spam / privacy vectors.

@peer  probably has a multitude of ideas and experience on improvements / things missing.