I love the idea of the arena, and have read your comments on the kickstarter page about its place in the game so as to not be more efficient then actual endgame.
Might I suggest using arena for alt characters…
So each account/season you only need to complete the story line once to get to endgame content.
Alt characters would have access to the Arena to grind up levels and starting gear.
When compared to games like poe or d3, poe makes you complete the story for EVERY character…and this just sucks when you have done it 3 or 4 times already in a 3 month league (not to mention the 50 times you have already done it. ). And Diablo removes the story completely…allowing you to jump into end game the moment you step into the new league.
I think a story play through is an important part of a new league/season. First of all the race of it is a huge part of the fun. Its kind of a competition. And it also allows you to fully display any additions/changes you have added to the story in new major expansions/seasons.
But upon completing the main campaign you should unlock the Arena for other chars on the account. Have it scale to the level of the character, and drop appropriate leveling loot. And just have it scale up to a similiar monster level as where the main campaign would leave you off. This would allow a streamlined, fastlane to get our secondary+ characters to the endgame. But would not be usefull for actual farming endgame content.
Just a thought.