I had to necro this post because I was thinking the same right now. It would be very fun to start with nothing than a grey armor and weapon and start opening chests to find gear.
The map could be shrinking over time (could be toxic clouds or something lore friendly with current expansion).
In order to avoid problems like rogues stealthing for life, tanks autohealing and being undefeatable, could be limitations to certain skills just for this game mode. The main goal would be to make each game more fluent and balanced, making every spec capable to stand against each other.
Also, I think the only viable composition to the mode would be at least 5 man teams with mandatory role choices. Could be like dungs (1 tank, 1 healer, 3 dps).
It obviously would take a lot of effort to blizz balancing the skills, etc. But I think it would be a very refreshing mode, casual friendly and potentially would take the attention of a lot of new players.
I really hope blizz think about this as an option.