Like I said, to answer that, we need to first come up with something new to add to tanking/dps in general. It's a very good question, and one that if you give me time, I could definitely come up with something. I'm sure the development team has been working on this.
Thinking about a SAM tank, how would it play? Surely there'd be countering and parrying involved, so this brings us to the interesting idea about countering. Then we ask, "is there a tank that already uses countering as a big part of their toolkit?"
Then we look at the tanks, and we find Vengeance, with Blood Price to a lesser extent since it doesn't really reflect damage. Then we ask, "is Vengeance a defining ability of WAR?" while also asking, "can we make a tank toolkit based around countering?"
Then you start coming up with ideas:
Ultimate Tank move: Counters all attacks for X seconds. (Similar in strength to PLD's ability, so should be a shorter duration given the benefit of damage.)
Sheltron-like Counter ability: Probably too strong, so one would have to come back, but the idea is there.
Building stacks with successful parries: Would be less reliable than WAR's stacks, so would need to make it worth it. While perhaps also making it not as relied upon as WAR's stacks.
Sheltron-like Parry ability to help build stacks: An alternate perhaps to the counter, but we should lay out all ideas in the beginning.
If you run into a wall here, you look at what other things could define SAM. Kiai has already been mentioned, so how could this be used in tanking? Well, one thing that is lacking in tanks right now is an AoE taunt. It's also a problem that every tank seems to have to use GLA as a cross-class to obtain it. These problems combined could be solved with an AoE taunt as part of SAM's toolkit, called "something Kiai" or whatever.
Stances are probably more linked with SAM than any other job's real life inspiration than anything, so I think we can safely say that they'd have the easiest time coming up with stances.
A DPS would likely follow a similar line of creation, with the main difference being that counter abilities would be of lesser importance than other stuff, like Kiai. I can tell you that of all jobs, SAM would have potentially the easiest time of having an abundance of names for abilities and stances to accommodate both a Tank and DPS, than any other job currently, so again, I think it would work well.
Does any of this make SAM a completely unique job than what we got? No.
Does it make for a unique play experience? Yes.
At the end of the day, making any job completely unique is going to be difficult because you run into many more obstacles, when all you really need to aim for is to make it feel like it plays uniquely.