I can't say how useful it'd be to split purposes with specs rather than introduce more trees like ACN. One would inevitably end up better than the other and become exclusively used.

On the contrary, splitting more "classes" into different Jobs would be delightful. I quote "class" since I think it'd be best used on DRK and the upcoming SAM.

DRK should expand so that Rielle takes up the path, but as a DPS and uses HP sacs and saps rather than enmity draws and MP, and teaches you how to become a BLD (Blood Knight) the DPS parallel to DRK.

SAM should be split into Samurai (Tank) and RON (Ronin: DPS). SAM would enfeeble and focus to mitigate damage, and RON would give up enmity and for damage.

It could be expanded into other jobs retroactively as well, like giving CNJ a WHM path and a GEO (Geomancer) Which could be the DPS or preferably Tank mage paths.