King Bardylis was as old as 90 by the time of the battle of the Erigon Valley and he died only a few months after his victory. If he had died only a year earlier, Philip II may have been able to come out on top of the Dardanians if they were led by the less experienced Cleitus instead of Bardylis. Realistically this probably would only delay the fall of Macedonia for a few years before the Dardanians came back and even if it represented the permanant expulsion of the Illyrians from Macedonia, it probably wouldn't have changed much when the Celts showed up.
But this is supposed to be a wank, so let's go further than that. Philip II was actually a rather promising young general before the disaster at the Erigon Valley and had already won some victories against the Thracians and Athenians before his death. He was also married to a Dardanian princess, so let's say that with Cleitus in charge at Erigon Valley the outcome is the opposite of OTL. The Dardanians are utterly crushed and their new king lies dead on the field. Philip presses his advantage and is able to use his wife's blood ties to Bardylis to push aside the other claimants to the Dardanian throne and take the Kingdom for himself.
A union of Macedonia and Dardania would be a force to be reckoned with but also somewhat unstable as the two kingdoms had been at war with each other for decades. Let's say Philip spends most of his reign consolidating control over both kingdoms. He reforms the military to integrate the best aspects of Greek, Macedonian, and Illyrian traditions. He implements efficient economic and adminstrative reforms to limit the autonomy of the nobles while providing them with real power in the army and bureaucracy to avoid alienating them. He asserts a dominant position over neighbouring Greek and Illyrian states by forming dynastic ties and through minor military campaigns which also serve to blood his new army.
Let's say Philip manages to reign for 30-40 years, giving him ample time to reform his kingdoms into an effective and powerful realm. For good measure let's say his heir also inherits Epirus through his mother's side. Let's call this lucky prince Amyntas after Philip's father. As king, Amyntas V would control what was easily the most powerful state in the Balkans. He could probably conquer all of Greece from this position so long as he avoided provoking the Persians - so let's say that he plays his cards right and does so. He takes after his father and spends the rest of his reign consolidating his gains, probably forming some kind of subordinate League to keep the Greeks in line. By the time the Celts show up in the Balkans, Amyntas would be an old man but his kingdom would be firmly united and able to repulse them entirely. This would mean that the entire Celtic invasion, bar those lost fighting the Macedonians, could be rerouted into Asia. Maybe we see a much larger Galatia?
From here we reach the Persian Civil War and things get much more uncertain. Perhaps the Macedonians could play a role as kingmakers during the war? Perhaps they could make conquests in Asia? A lot depends on if any of the Persian satraps could unite enough of his peers behind him to found a new dynasty like in OTL. Historically Egypt, the Levant, and Mesopotamia almost broke free from Persian control during the war, maybe if this super-Macedonia supports them they could throw out the Persians for good? If you wanted to take this wank to its extreme, you could say that the Macedonians take advantage of Persian instability to conquer the whole Empire, but I think that we can all agree that I'm already pushing the limits of plausibility and that that would be straight up ASB.
