My theory is that basically any job will be able to join a party as any role.
Will every job be ideal in said role? No, absolutely not. You're still never going to have a WHM or AST tank a mob, for example, but could a Dragoon tank a mob with a wider variety of enmity skills available to them? Potentially yes. Doesn't mean a DRG is ideal in that scenario, however. This would also open up WAR/DRK/PLD as DPS spots in a party. I don't think its crazy to suggest that with the right DPS cross class skills those jobs could be beneficial to party comps (especially say PLD as support/damage with Clemency, DV, Cover)
I think this also dovetails with the release of the new jobs. I'm at this point convinced that the two new jobs released will be Red Mage and Samurai. If my theory on how they're reworking the skill system holds, they aren't simply releasing two jobs, they'll be releasing four. Red Mage as a DPS or Healer, Samurai as a DPS or Tank.
By moving cross class skills to role as opposed to a very narrow subset of classes, they'll be opening an opportunity for people to develop their jobs more to fit a specific playstyle. If you don't want to play a Tank SAM, you can just be a DPS SAM by changing out skills - almost as if they're choosing this route instead of adding skill trees. Where a tree would make a player choose DPS SAM or Tank Sam, this system would allow a player the option of both.
It'd also ease design somewhat, because they could add new skills for both (say, 6 new skills per job on the way from 60 to 70) but these skills would be more unique to the role that you're currently playing as.
Example: Perhaps RDM has an ability at level 52 that gives all its healing potency a buff for 10s, and at level 54 it has an ability that gives its damaging spells an additional damage over time effect for 15s.
This way they can add skills but keep the number of skills per role lower so they don't have what I'll call 'rotation creep' where by the end of a 3.x or 4.x you have 40 buttons to push/keep track of. I think it'd also allow for more creative uses of jobs going forward, with people trying new or different strategies because now the number of party comps increases significantly. It'd also have the side benefit of easing future job design.
Just my current thoughts.