The only way to do a "ranged" tank in the game would be to use a pet-like thing that the mob wails away on while the tank is someplace else.
I can't imagine that'd work too well...but it sounds kinda cool in concept.

I think SAM can make a pretty interesting an unique melee tank with counters and perhaps some form of partial evasion.
I also think it'd be cool to have a melee-magic tank. Shield and wand!