In theory it sounds like a cool idea, it just seems like the job system would have to be revamped to support more versatility.
What would be the main difference between MNK and PUP? A pet and a few different abilities? Are 5 abilities enough to distinguish them?
If this were to happen I would hope to see more job specific abilities to give each separate job path a distinguished role from the base class, rather than just a HQ class.