There's a whole conversation that could be had on that. There's two approaches available for BST as a tank. One would be similar to how SCH is built with the pet serving some mitigation function. The other would be the BST riding their pet into battle as the "tank stance", combining the stats of both. I'm partial to the latter, though the low hanging fruit of why that wouldn't work is due to Roegadyn and how huge they'd be riding on top of a wolf/bear/turtle. Imagine a party with a SMN that has summoned Bahamut and the tank being a roegadyn BST riding a bear; you'd probably be able to see very little of what's going on.
The former runs into issues of pet responsiveness and how the current system could screw over a BST tanking when trying to mitigate a tank buster or high-damage phase.
I won't dent that it would take work to make BST a reality, but that's what the devs get paid for. Using XI as a template is not a good idea, since XI's BST was a mess by design and didn't have much that made it stand out aside from charm and jug pets.
On DNC: First, here.
Second, a lot of the FFXI stuff would need to be retooled for it to work here. Fan Dance would most likely be a defensive cooldown rather than a stance. Steps could go either way as debuffs or as buffs that give the DNC an advantage over whatever it is they're tanking. And I agree that Healing Waltz would be a no-brainer in terms of self healing.