My guess is that Astrologian will swap between WHM/SCH healing-styles, utility skills changing depending on stance and/or cards and that they will have less consistent/reliable DPS-skills that do good damage but are more MP/time intensive than either. Shields probably won't stack and will be somewhat weaker or more "unreliable"/depend on the card mechanics. This would mean that Astro "fits in" better but does not replace either as the other two have more specialized healing abilities.

I also feel that DRK will be different, in that *it* will occupy the extreme "damage-tank" end of the spectrum, with WAR sitting in the middle (with more mitigation) between DRK and PLD.

MCH will probably be less AoE centric amongst the ranged DPS, and will be better for focusing on 1-2 enemies compared to BRD/BLM, probably occupying a damage spectrum between the two and focusing on 1-3 enemies. Could also go all-out and make them the "physical AOE" expert, usurping BRD for that.