Adding a new tank and healer can work, but they need to fix the mistakes of their previous addition of new jobs:

1. Drk/Ast/Mch starting at 30 is way too low. They should have started at 50 so that anyone that wanted to go through the expansion with the new jobs at 50 could have been able to. They repeated the mistake with rdm/sam by starting them at 50 instead of 60. Dear SE, NEW JOBS NEED TO START AT WHATEVER THE PREVIOUS CAP WAS. Rdm/sam should have started at 60, Drk/Ast/Mch should have started at 50.

2. Drk/Mch/Ast were underpowered as hell when they were implemented and while they weren't necessarily bad, they were nowhere near as strong as they should have been.

3. They need to be more fun to play. They did a better job of this with Sam/Rdm as both jobs feel great to play as soon as you pick them up with unique mechanics that are easy enough to grasp and all of their abilities after build on the core concepts. As opposed to Drk/Mch which used radically different mechanics (MP management with constant MP loss, certain stance locked abilities, ammo, RNG on combos) and then they got some pretty drastic changes in gameplay style with things like the blood and heat gauges and the nerf to Blood Price MP regen.

TL: DR version:

1. New jobs in next expansion need to start at 70, this is not debatable.
2. The jobs need to be strong enough to make people want to play them.
3. Their playstyle/gimmick needs to be easy to grasp and figure out while also being rewarding to whoever masters them.