I think this is very interesting, would have to be some limitation to it though. To better organize it:

[Job Ability]
Mimic Ability
LV.5 Mime
Duration: Instant Recast: 1:00
Mimics the most recent(or as an alternate option; One of the 3 most recent) Job Abilities performed by selected target.


[Job Ability]
Mimic Cast
LV.15 Mime
Duration: Instant Recast: 1:00
Mimics the most recent(or as an alternate option; One of the 3 most recent) Magic Spell performed by selected target.

[Job Ability]
Mimic Special
LV.30 Mime
Duration: Instant Recast: 2:00
Mimics the most recent(or as an alternate option; One of the 3 most recent) TP Abilities performed by selected target.

[Job Ability]
Mimic Pet
LV.50 Mime
Duration: 3:00 Recast: 10:00
Mimics the most recent(or as an alternate option; One of the 3 most recent) Pet summoned by selected target.

[Job Ability]
Impersonation
LV.1 Mime
Duration: 5:00 Recast: 2:00:00
Allows you to gain full benefit of selected target.
{Example}Say can use on an HNM such as Fafnir. Your Character will change into a MiniFafnir and you will gain full access to all of it's traits, abilities, and special attacks for 5 minutes.

This requires more thinking, but this is a rough idea of how the job "Mime" could be made. Please leave feedback or feel free to discuss how to alter these examples. Thanks!