That would be cool. I think it could be pulled off with what exists in NIP-52.

1. Alex blocks off his unavailable times using kind 31923 time-based calendar events and/or kind 31925 RSVPs with the fb tag set to busy.
2. I query for Alex’s busy times to extrapolate his available times, and create a kind 31923 time-based calendar event for the appointment I want to schedule. Use the p tag to invite Alex.
3. Alex accepts the appointment request with a kind 31925 RSVP, with an additional e tag of the specific appointment.
4. If Alex doesn’t agree to the appointment request or wants to alter it (e.g. wants to limit the appointment to a specific duration), he could refrain from RSVPing to the calendar event sent his way, but rather create his own calendar event and tag the original requester.

We don’t have any kinds yet that allow someone to specify how long their desired appointment openings should go for or when their desired appointment openings should take place. But I’m sure we can make that happen.

@peer Does cal.com have any plans to integrate with Nostr? I know you weren’t in love with the NIP-52 spec.