I think Mediator is a very unique support job and would be very welcome! The only trouble is that I would recommend against any Crafter/Gatherer buffs and give Mediator some offensive capability for solo potential instead.

As for Mime... Mime is basically the mage analog to Freelancer. Which is to say... either the worst job in the game or the most overpowered job in the game, depending on skill level of other jobs.