So the Fort subs I have seen the most and what I like about them.
Landy: Power and Mind Break, Swordbreaker and an Attack Boost if you want it.
Dancer: Fan Dance of course. Burst saver and quick step are good. I love the Refresh and Freedom dances. They dramatically shift so many annoying and difficult fights to your advantage.
Medic: Fort can pick up Patch Up to save your Arc/Medic some points (my Arc/Medic is always low on points it seems). Also another source of Revive and Auto-Revive is great. Plus can take the status cleaners and with the Fort passive that cuts status time (maxed out Binds and Status pretty much never stick) Fort/Medic can be a great cleanser.
as a dancer the heal dance tree and speed boost you save your team from many binds and negative stats that way...also regen waltz makes the front line more unkillable if you have the space left in your buff slots...speed boost and fan dance are bugged so pick one or the other...
as a landsknect, after setting up strike guard and taunt you can use mind break/power break to weaken enemies attack <good thing is it doesnt use a buff slot it adds a debuff to the enemy so it should stick> swordbreaker is awesome too making his line even more unkillable it procs pretty often...the dualwielding for defense eh...not really good...
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for the shorter term i went with medic, because its awesome having someone with great defense who can also revive and get back tp. Its probably not great for end game, but hey, rest only costs 2 levels.