I'm not sure about healer. The way this game works, I highly doubt we'd see a healer that uses a melee weapon, where said melee weapon is expected to be used often and effectively, yet the class is still a healer. So if RDM is a healer, they wouldn't do much with their sword.
I would absolutely love an RDM centered around either a traditional rapier or a gunblade. And in either case, I strongly doubt that "combining white and black magic" would manifest as "it gains several spells from WHM and BLM and uses them". Rather, it would have its own skill set that draws from the role and feel of those classes - i.e. some support or healing abilities, some magic-based damage abilities. But you wouldn't just straight-up cast Fire 3 or Cure 2. At least, I'd be surprised (and disappointed) if that was the case.
With either weapon, I could see it working well as either a tank or DPS. As the former, it could be essentially an evasion tank, using a fast, flourish-y sword style, with the animations for its damage mitigation moves involving a lot of dodging and weaving - successful use of a move might have the RDM spin out of the way and then dive back in for a quick strike. That kind of thing. As the latter, the idea of elemental sword spells (mentioned by at least one person elsewhere in this thread) could be interesting. It could be a set of spells you have to cast prior to using your melee weaponskills; these would have much shorter cast times than classic "spells" (i.e. WHM/BLM), and you'd apply one based on what you wanted to do, with the different elements either having BLM-like effects (Thunder adds a dot, for example) or something new.
I REALLY doubt that this game's environment, particularly at high-levels/end-game raiding/etc., would allow for a class that is truly a jack of all trades, master of none. If you had a class that could take damage to the face and survive better than any non-tank, but still not as good as a PLD, and could cast offensive magic from a distance better than any non-mage, but still not as good as a BLM, etc etc...
No one would ever want that class around. They'd just take the better classes for each category. So I don't think we'll see a true "hybrid" in that sense. Something that is a hybrid in some other way (using both MP and TP, or having an even mix of melee vs. ranged skills, for example) is entirely possible.