Magus loses an Arcana and gets less Arcane Pool points, so I'd try for something similar to that instead of wealth by level, which is pretty vague and always depends on the DM anyway.
The Arcana would be roughly equivalent to a Rage Power I think, but the Arcana Pool nerf is more problematic; you could say that a Barbarian gets less rounds of Rage gained by level (not a fan), but besides that I'm not sure.
the arcane pool the magus loses, is compensated by the pool of the black blade. just make it a feat and call it "Ancestral Weapon." or something like that. give it enhancement bonuses that scale with level and so many points for special abilities.
I think a feat would be too cheap for something like that...maybe a series of feats?
a general feat is a pretty fair tax for a single scaling magic weapon. it delays the feat chains of entire builds by 2 levels. makes 1 less rage power a barbarian may choose. and considering how long one spends leveling, the sting will be felt.
The core rulebook DOES have rules for pricing intelligent magic to start with. Making it more powerful as they level would simply be a matter of seeing how their WBL increases upon levelling and assign a new ability or bonus of similar value.