and after all i think the OP is only fearing DK to be tank because he belives WAR was actually forced into being a tank, while in older FF games WARs & Knights have practically shared the same role, just with different skill sets.
i'm not saying it's impossible for DK to end up being a tank, but it sounds rather delusional thinking about the DKs role.and that is to be a suicide bomber that casts black magic, simple as that