IOTL, the powerful Thessalian tyrant Jason of Pherae, who may have had pan-Hellenic ambitions and ideals, was assassinated in 370 BC and replaced with his largely incompetent son Alexander, who was eventually overthrown by the Thebans. What if Jason had not been assassinated? He had a powerful army and seems to have been a proto-Philip of Macedon. Could he have unified Greece? Would he?
So I actually used this as a POD for a now defunct timeline of mine. As you said Jason was basically a proto Phillip in many ways-an efficient professional army that was leaps superior to any opposing Greek force in quality. Already he was sort of playing a role of kingmaker in Greece and had played a not insignificant diplomatic role in the conflict between Thebes and Sparta. As was en vogue at the time he latched onto the Isokratic idea of pan-hellenism as a way to combat the real threat, the Persians. Like Phillip would later, he talkes openly about unifying Greece in an endeavor east to take on the Persians-this was likely his ultimate goal.
Now achieving that is still hard. At first glance Jason's control over Thessaly as Tagus was much less centralized than Phillip's over Macedon as king, but Macedonian kingship was not very autocratic in any case and before Phillip Macedonian rule was not very centralized in the slightest, with Phillip having to spend significant time bringing semi-independent dynasts in the periphery into line. There's arguably much more potential for Jason to consolidate power in the more urbanized Thessaly.
Aside from intra-Thessalian politics, a major obstacle I see to Jason's goals beyond dominating Greece is his very strength, which is his reliance on a mercenary force. This might not be as big a problem as I'm making out, since its true that mercenaries could be quite loyal and effective (the Persians used Greek mercenaries effectively, as did Alexander in not insignificant numbers). But they're also prohibitively expensive in a way that the cheaply armored sarissa pike phalangites of Phillip and Alexander were not. Can he equip enough of them in enough numbers to be able to take on the might of the Persian Empire? Perhaps, if his goal is more limited like Phillip's was.
There's potential for him to take advantage of the collapse of Macedon to his north for Jason to gain territory and cheaper manpower if he plays his cards right.