No one in the industry believes that open rate is 100% accurate, or even 50% accurate. It doesn't need to be. It's a directional measure: Which subject lines have better open rate, how does open rate decline as you send more frequently, which mailing lists or demographics have better open rates to the same email, etc.
"opens" for email messages have never been super accurate, since downloading images isn't on by default in more than just Gmail - your desktop mail client may have the same settings - who knows?
Because of this, any real interaction with an email message - where the user has to interact with the content, could then be counted as an, "open". If you get any other interaction with the message - say a user clicks a URL (that is also tracked), you just look and see if an, "open" was also recorded for this message, for this user - and, if not: record an open.
You'll still get instances when opens happen in messages, that aren't tracked, since no other interaction is done - there's that inaccuracy. But, if no other interaction is done, might as not count it as an open, anyways. Perhaps we should rename, "open" track as, "was this person, at all engaged, in the slightest?"
You can also then just give that a ranking: opened, clicked x amounts of links, followed through with a sale, DIDN'T unsubscribed - good rank!