Yeah I definitely like this idea.
Ranked 1 v 1 could be called duel and might happen in coliseum, while you could chose your oponent by setting up 2 members party with him or going random trough duty finder.
Unranked 1 v 1 could be called a spar and might happen on some proving little wooden arena.. somewhere.

Or just give us a right click option + option to ignore duel requests. Whatever. Just gimmeh PVP!