I'd also like detailed search of contacts (from FastMail's page):
You can search for a user by these fields: email addresses, phone numbers, online contact (website, Skype handle, Twitter handle, etc), notes, nickname, company, title. Searching isn't case sensitive (searching for a matches a and A and any accented versions such as Ǻ).
Searching provides immediate feedback, with all contacts who match being shown instantly, narrowing down the results as you continue to type.
Tasks search
IMO https://tasks.org/ does this good although on Android app. It's open source.
Global/multi-journal search
Search across any calendar, contact list and task list, respectively, using any journal-type specific criteria. So searching multiple calendars for a "location"/"where" field will be possible, while perhaps not when searching only contact-type journals
Search globally across any/all types of journals using free-text ("match anything anywhere")
Be able to select the journals to search within within one journal type (eg only calendar 1 & 2 or only task list 1 & 5) and use journal-type specific criteria for this search
Be able to select journals across journal-type (e.g. search across calendar 1 & 2, address book 3 and task list 4 & 5) and search these using free-text ("match anything")
Calendar search
I'd like to be able to search a calendar for an event/group of events. Why re-invent the wheel? See good search options here: https://www.fastmail.com/help/calendar/calendarsearch.html (if link dies, then check here)
See also the email search function descriptions on how to filter by date ranges etc: https://www.fastmail.com/help/receive/search.html (if link dies here)
Contact search
I'd also like detailed search of contacts (from FastMail's page):
Tasks search
IMO https://tasks.org/ does this good although on Android app. It's open source.
Global/multi-journal search
Search across any calendar, contact list and task list, respectively, using any journal-type specific criteria. So searching multiple calendars for a "location"/"where" field will be possible, while perhaps not when searching only contact-type journals
Search globally across any/all types of journals using free-text ("match anything anywhere")
Be able to select the journals to search within within one journal type (eg only calendar 1 & 2 or only task list 1 & 5) and use journal-type specific criteria for this search
Be able to select journals across journal-type (e.g. search across calendar 1 & 2, address book 3 and task list 4 & 5) and search these using free-text ("match anything")