I would agree that there is plenty of space to offer additional benefits to a Platinum tier, without diluting Gold or Silver benefits. Though some things may be harder (too hard) to implement than others. For example anything that has to do with non-390 ticket stock Aegean will never be able to touch/re-issue as they do not own the ticket. But there are plenty of other benefits that could add value on top of Gold, many good ones mentioned above.

However if this were to be a tier on the alliance level, then the majority of benefits would likely be connected to those alliance benefits that would apply throughout the alliance with all member airlines. Meaning they would likely not be specific to Aegean services.