Shoggoths are already in the game -- think Othernesses.
Not sure that I'd agree that every path requires a super-summon. The problem is to balance nations, not to balance paths; if one nation has a very strong early game with cheap F9W9 but cap-only sacreds, and another comes into its prime during late-game due to summons et al, and another is best suited towards lavish but expensive scales for cranking out high-quality conventional armies during a good midgame -- that seems reasonable. No need for mirror-image balance.