Have not really put to much thoughts about it. It should be limited to damaging specc's only. And hybrid classes that can heal should be limited in one or another way, or put in some counter that add damage done vs healing made and the one with highest score win.
Class restricted as in warriors vs. warriors, warlock vs. warlock etc.
To make it diffrent from a normal duel then there would be 3-5 diffrent items you can buy, but only one will be possible to use/fight (stun-flask, fear-flask, jump-back flask etc) that will ofc only work in 1vs1.
All fights start with you having a 15 second "invisible" and "bubble" mode. Can't take damage, can't be seen or tracked, just so you can run and hide on your spot or whatever.
Rating from it will be stand-alone from the normal arena (but give honor points), and rewards will be worse quality than actual arena-season. But the end reward will be some title like...well, whatever sounds and fit good for your class.
A system more close to how it was working in vanilla WoW.