Today we have 4 criteria in Arena: Specials, Hell Forge, Epic and Max lvl.
I am suggesting to add a new one - Set items.
This will allow using so many common, rare, unique items which are not part of any Set to be used in arena competition. It will positively impact on auction house /buying-selling ordinary items/ and even FSP usage /forging need/.Today arena players mainly use legendary sets and arena became one hit preset process for them.