It would be very helpful to be able to create customer searches that can search against contracts as well as recurring invoices based on what line items are included.

For example, I want to send an email blast to all customers on a certain level of monthly service agreement. Ideally, I would create a search filter that says "find any customer with a [recurring invoice / general subscription / ...] with [this/these product(s)] as line items".
Or, "find any customer has a contract with [word/phrase] in the name."

Some basic boolean logic would help too to allow multiple inputs separated by an AND/OR.