WAR is the best DD sub for DNC purely due to Berserk. We don't get the Double Attack because Saber Dance seems to overwrite the trait. Dancer doesn't gain much from any sub relative to other jobs, though.
These are the problems I see for Dancer along with the solutions and justification:
1) High Job Ability delay (the pause in actions after using a job ability) limits our playstyle.
Adjustment:
* Reduce both phases of Job ability delay by 1 second specifically for Steps, Sambas, and Flourishes.
Justification:
Using steps and flourishes costs you multiple attack rounds and commonly is only used to gain TP, which you can also gain by just taking those attack rounds. You might gain less TP by simply attacking, but you're also doing damage at the same time. Due to this, it's not actually worth using Steps and Flourishes unless you're skillchaining or solo. Because you can pretty much only skillchain when you're solo, it's safe to say that Dancer playstyle breaks down when you're in a party. Not good for an alleged support job.
2) Samba viability problems: Sambas are impractical in current events, Sambas other than Haste Samba suck, Haste Samba doesn't matter if you're already capping delay, and in high buff situations there's no difference between DNC main and DNC sub using Haste Samba for any job (the last 5% is irrelevant).
Adjustment:
* Increase Haste Samba for DNC main job to 10% JA Haste base without merits and leave /DNC Samba at 5%.
* Change Haste Samba merits from increasing Haste to adding 2% Haste per merit level to Drain/Aspir Sambas.
* Fix Spheres so that they affect the user.
* Change Sambas so that they're a sphere that gives their effect.
* Change Drain/Aspir Samba so that they calculate maximum Drain amount before forms of delay reduction.
Justification:
The 80% Delay cap can be hit with 25% equipment Haste, 43.75% Magic Haste (the magic haste cap), and 11.25% Job Ability Haste/delay reduction. This last 11.25% is covered by Martial Arts/Dual Wield for 1H jobs and mostly covered by Hasso (10% of it) for 2H jobs. That leaves the last 1.25% for Samba, and both /DNC Samba (5%) and 5/5 Haste Samba DNC Samba (10%) can cap it. This is really pretty insufficient, and means that DNC isn't going to be useful in high buff situations.
Instead of fighting that, I would recommend pushing towards DNC being a buffer/DD hybrid for lowman events. The problem with lowman events at the moment is that you're generally fighting level 75 content and everyone is fighting a different monster, thus not getting Samba. If Samba was a fixed-Sphere effect then this wouldn't matter, and allowing us to put Drain or Aspir on top of that would be very nice.
3) Our Flourish recast timers are too limiting.
Adjustment:
* Reduce Wild Flourish to a 10 second recast
* Eliminate Building Flourish's recast timer, or switch it with Desperate Flourish.
* Reduce the Flourishes III options to a 1 minute recast
Justification:
Wild Flourish is currently the competition for Reverse Flourish especially if the monster has <100% HP, but I find myself unwilling to use it because Reverse Flourish's damage potential is ultimately so much higher and I may want to use that instead during the recast. If the recast was less of an issue, I would be more willing to use it and my playstyle could be more fluid. Similarly, Building Flourish currently sees no use because of JA delay and the fact that it can't be chained together with other WS buffs (like Wild Flourish and Reverse Flourish). If you eliminated it's recast timer, we could do that.
The Flourishes III reduction is less important, but more just a "why not?" kind of suggestion. The only useful flourish from that category at the moment is Climactic Flourish, which has the shortest recast. Why not just reduce all of them to 1 minute so people can use them more freely? It stinks to have a whole category locked out for minutes if you use one of them.
4) Our Waltz abilities share the same recast timer, and this is terrible if you're actually trying to heal.
Adjustment:
* Reduce Curing Waltz IV to a 12 second recast and Curing Waltz V to a 14 second recast.
* Split Waltzes into two categories (odd and even), with Healing Waltz in the even group.
Category 1: Curing Waltz, Curing Waltz III, Curing Waltz V, Divine Waltz
Category 2: Healing Waltz, Curing Waltz II, Curing Waltz IV, Divine Waltz II
Justification:
There are two ways to look at Dancer as a healer. First, you could say that because their job abilities are nearly instant there should be a substantial penalty associated with them in the form of a recast timer. Second, you could say that because their abilities cost TP they should be self-limiting (due to your TP gain) and don't require any substantial penalty beyond that. I am of the second camp. At the moment HP healed/minute is higher than possibly usable for healers like White Mage, so it wouldn't be game breaking if Dancer could spend all the TP they make and increase their HP healed/minute. Splitting Waltzes into two categories still leaves them somewhat recast dependent while allowing us more variety with our healing.
5) Samba duration is too short.
Adjustment:
* Increase Samba durations to 3 minutes, with AF1+1 head adding another minute and Saber Dance merits adding 15 seconds per merit level for a total possible duration of 5 minutes.
* Double Samba TP costs.
Justification:
Reapplying buffs is tedious, especially when we have essentially no reason to switch between the buffs. Making Sambas cost twice as much and last twice as long reduces the annoyance for the player while still providing the same penalty.
There are more adjustments to be made (like making Flourishes III useful), but I'll stop here for today because it's time for post-Thanksgiving Dinner.