New jobs upcoming, new tanks?

[Gamemako]

It could use rune blades which could be like those swords with 2 ends

Thief Swords, as in Zidane's weapon from FF9? That would be an unusual choice. They only existed in that one game, and are in name associated with thieves. Are you referring to something else, though?

They need a really wanted class to be a tank so people play it (SAM/DRK). That would make the most amount of sense.

Oh, that's easy. Add lots of spikes to the armor and blam, people will play it.

B> Flintlock to Gunner or Cannoneer

Ranged tanks are difficult to implement, particularly in a game like FF14 where enemies typically have no means of closing distance. I think any tank in ARR is going to need some specific melee capabilities for balance reasons. You could at least in theory implement any gunner this way (short-ranged PBAoE effects, with lots of jumping and spinning), but it would be odd. I believe a gunblade would be more suited for the task, but I'm not sure we'll ever see one due to the association with Garlemand.

if they're adding mystic knight, i would like rapier as their weapon >.< evasion and barrier tank so that they will not take a huge hit if they fail to dodge

That would more or less how you'd have to set up any evasion-focused tank. Evasion is fine as a long-term mitigation mechanic. However, it is completely irrelevant for bursts, meaning you need to include an entirely separate mechanic for surviving those. That said, you're always going to need huge eHP pools for your tanks. It is unavoidable, more or less. Any character with less than the +25% on both current tanks is going to get flattened too easily.

as for dark knight, how about using hp sacrifice as a source for increasing the defense

I believe that would be nearly impossible to balance. The amount would be a flat reduction with the benefit scaled to content -- it would effective scale inversely to how current WAR does. If it is balanced to 8-player content, the HP tradeoff would be terrible in four-player content. If it is balanced to four-player content, then it would be overpowered in 8-player content. Since it would have to be balanced as gimped in 4-player or overpowered in 8-player content, it would effectively preclude any four-player challenges.