Commander format
MTG: Duel of the Planeswalkers 2012 had 4 player MTG... Back in 2011.
There's not a single reason in today's age of technology we should be behind what we were 13 years ago.
Introducing the Commander format to Arena would increase Arena profits because people wouldn't have to flock to outside resources like Untap, Spelltable (which I understand now is backed by WOTC) and other places like that just to play Commander.
I know personally, I would spend more money on Arena playtesting Commander decks, but with the lack of Commander format (Brawl is a poor imitation and everyone knows it.) and with the platform missing a very large amount of the common cards within the Commander format, it makes it hard for me to justify and has pushed me to almost abandon the game altogether, as I primarily play Commander now. I have been a player since MTG Arena was first introduced.