I thought that the ACN -> SMN/SCH split idea was a cool one, and I'm a bit bitterly disappointed that SE decided to kick it to the curb. Folks here on the forums came up with some pretty creative ideas about how currently existing jobs could be branched into different roles, even taking into account their supposedly role-centric abilities. One such suggested idea I thought was cool was a DRK that branched off of GLD, with abilities whose DPS scaled with the amount of enmity the DRK had, so the DRK would be constantly working to maintain a position second or third on the hate list, just behind the tank(s). Another idea I saw tossed around was a healer branched from THM, whose spells either added shields or regen depending on whether they were using Astral Fire or Umbral Ice; swapping between the two to use the appropriate additional effect would be a core mechanic to the job.

The devs decided, though, that creating new jobs from whole cloth would allow for more freedom, and even if I don't personally like it much I can't really disagree with that assessment. It does sacrifice one of the things that would have given FFXI's job system flavor, though.