But these already exist; it's just that the first one is called BLU. The mechanism for having a job that takes a completely different role is *job change*. It's available from any moogle. There's no good reason to have one individual job pulled in so many directions at once. And there's no indication that the devs ever intended RDM to continue meleeing on very high level opponents (outside of situations like campaign, where below-average melee is ok).
RDM can't out-melee melee jobs that can't do anything else; that would be like a DD with shadows out-tanking a tank job... ok, bad example, but hopefully SE will fix that eventually.
Best-in-game enfeebling and enhancing, second-rate but serviceable nuking and healing, below-average melee is where RDM has always been, and where it's almost certainly going to stay. And below-average melee isn't going to be terribly desirable in group situations when there are lots of melee-focused jobs.
Tranquil Heart is lol for whm, they already have cure V+ which have massive -enmity built in, but RDM and SCH definitely can get unwanted enmity from cure IV spam, especially when lowmanning with something like THF/NIN. (Let alone Accession Cure IV.)