Add two more search criteria to CiviEvent search block (Find Participants and Advanced Search:Events section):
Event Type: [____________^]
(select field, place on same row as Event Name - right side column)
Role: [ ] Attendee [ ] Speaker ....
(checkboxes, dynamic via configured Participant Roles, place in new row below "Status")
If either field is empty, no additional filter is placed on the query (i.e. we find ALL event types, ALL roles)