This is in addition to expensive activated abilities on Qiqirn Merchant, Crystal Fragments, Gran Pulse Ochu, Ride the Shoopuf, and several rares
In general, when looking at power level, I considered the overall game plan, the synergy of the 99 with that game plan, things like interaction, ramp, mana base and card draw, the commander, and the power of the remaining stand-alone cards
Sneaky answers include things like Kheru Spellsnatcher , who can steal a spell for its morph cost, and Trygon Predator , who can conceal its true identity as a 2/2 face-down card until it's time to spring into action to destroy an artifact or enchantment
I played with Thopter Engineer a lot and am happy that a similar replacement is here
On the opposing end, Black cares only for power and pursues it relentlessly