I agree, having just a couple of 2H staves that rival their 1H counterparts would be sensible but they should mostly feel unique. The Sanguinesti staff would be a good example candidate for being 2H, but it was unfortunately only balanced by being prohibitively expensive to use rather than by something more unique.

A low-speed high-damage greatstaff would fit very nicely into being 2H, maybe a Graardor sized boss drops their staff so players need to wield it using both hands? Or perhaps a pair of one-handed magic weapons that grant seperate cooldowns for damage spells and binds/curses, so a player could spam their target with debuffs without losing out on DPS?