What is the optimal Burgundian strategy?
No "Let's build my own kingdom" non-sense to begin with. It would force Burgundy to fight on too many fronts.
His first focus would be the...*sigh* "Spider King" (XIXth called) : he's the more powerful threat to a Burgundian hegemon on the region. Building alliances with unhappy nobles,
Ligue du Bien Public-like.
He would need to be as, apparently, conciliant than Louis XI : ready to concede advantage, if not lands or revenues, to his allies or soon-to-be.
I stress this point : the goal there wouldn't to increase his principality power, but to weaken royal power, would it be at the (limited) cost of his own.
Eventually the better strategy in a first time would be to re-impose the Bourguignon political hemegony on Northern France, early XVth style. Without this, Louis XI would still beneficy from more legitimacy and ressource than him.
The second issue is the extreme disparity of his holdings : Netherlands are working autonomously on several matters (and are wary of their freedoms), and while administrative and political uniformisation is required, Charles have a clearly less unified background than Valois to work with.
Again, it means a lesser fiscal pressure : no shiny and glorious wars as Charles did IOTL (but I'll point that it would cost him on the other hand, as such behavior was extremly "fashionable" in Burgundy where heads were full of knighthood tales) but more political and subtle touchs.
Critically, no action in HRE before France is dealt with, except to make compromises and eventual concession to gain support or at least non-intervention at Louis' behalf.
Anything contrary (but it's bound to happen, Charles couldn't ignore Germany even if he'd want to) is eventually a gain '(of time, of ressource, of diplomacy) for the king.
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That said, it would be (I stress it) completly out of the cultural background that existed in Burgundy at this time (and arguably in many of the last important feudal principalities in the kingdom).
What I tried to point would be an ideal stance, but realistically, would have to deal with the mentalities (and even without considering that, doesn't counter-balance entierly the more favourable ground for Valois)