We also have our one, and likely only attempt at specced jobs already in the game. Play Arcanist? You have your choice of playing as a Healer or a DPS. You "respec" by changing job crystals. What you're suggesting sounds no different, except that you'd have three job crystals instead of two.

The devs have made it quite clear that they feel what they did with Arcanist was a failed experiment, and that they will never do so again. I don't see a lot of hope for your idea.

Best get used to the idea of RDM being a tank, healer, OR DPS, not any combination of these. The best you can expect as a nod to RDM's multi-role utility is that they may get a minor curative ability if DPS, or a handful of low-damage nukes if healer.

And tank? RDM never really was a tank, aside from a brief period in FFXI where deeper understanding of the enmity system allowed players to hold hate by casting specific spells that players' experimentation had revealed to have higher-than-expected enmity - something SE quickly put the kibosh on by removing nearly all the enmity from said spells. "Classic" RDM in the Final Fantasy series was a combo of nuker, melee damage, and healing; tanking was never part of it.