So this is a pretty simple idea for a tactical item.
I like the old space missions, and the item you had that toggled you between a firing damage buff and a shield Regen buff was pretty cool.
I think it could be interesting to have a "stance change" toggle ability that would buff your damage or decrease you GCD when toggled off, but buff your defense somehow when toggled on.
This could be useful in bosses to get extra damage in when burning, but get some extra survivability when the boss is doing the burning. The buff to damage output would reduce your defense, and the increased defense would come with reduced damage output. Basically, you'd become more effective, but only at the cost of switching your stance with the new abilitiy
Also, I realize this would be extra work, but if when toggling defense my toon held his lightsabers backwards (think the Force Unleashed style), that would help make sure I don't forget the stance I'm currently in (and look AWESOME).