We've started small and deep instead of broad and shallow. Here's the honest list.
Google Calendar (two-way sync, conflict checks, event creation), Google Meet (auto-generated link in every booking), and Razorpay (UPI, cards, netbanking for the Considered plan).
Roughly thirty percent of Indian corporate users live in Microsoft 365. Outlook calendar sync is in active development. Azure app registered, OAuth flow being built. Estimated arrival: this quarter.
After Outlook: Apple iCloud calendar (CalDAV), Zoom auto-link generation as an alternative to Google Meet, and eventually a public REST API for embedding Kaien into other tools.