For my opinion, it makes no sense to create such a FOM.
There are many, many different applications for opamp with different requirements.
On the other hand, there are many performance parameters describing the opamp properties like:
GBW, stability margins, slew rate, offset, offset drift, port impedances, power consumption, rail-to-rail properties, supply voltage range (lower bounds), sensitivity upon capacitive loads, ...
For most applications only one or only some of these parameters are important.
Therefore, I consider it as not appropriate to define such an artificial "figure of merit".