For some of my events I need to add a custom field which I want to make required. For other events I don't want to include this field and I use different profiles to deal with this. My first idea was to make the field required in the custom field definition and this was fine for online registration, but on manual registration it was still required for manual registration for events that don't use this field. Looking in SE I found a couple of questions Manual Event Registration problems and Manual Event Registration problems which help a lot. If I don't make the fields required in the field definition but make them required in the appropriate profile stops me having to enter them for manual participants when not relevant.
But then someone can register a participant manually without putting filling in the field that needs to be there. I can live with this but its not ideal.
From the questions referred to above I could restrict the fields to only one type of event, but that means having event types "course" and "course with special field" which is clumsy and probably confusing, making the event type not very useful for report filtering etc. Also I can't retrospectively apply this to an event that is already open for booking because I need to create a new custom field.
Any other suggestions?