Seems like a future mages post!
I would imagine like any money making venture or company as long as customers are paying, buying expansion sets and other new content they will keep putting it out.
I would imagine AW sales and marketing force are looking not just for future expansion/content releases but also diversifying in selected areas of growth opportunities.
Increasing and maintaining player fan base must rank extremely high though. Like was mentioned you don't want so many traits, conditions, effects the game gets confusing while still adding diversity of content.
The best way to do that is the different mages or combinations of major classes. Like I would LOVE a holy/dark mage or elemental mage or things which have already been discussed. I'm definitely looking forward to the Druid/Necromancer expansion no matter what traits/mage abilities or cards are in it almost. I still play EverQuest after 13 years and I've always been a Druid so anything Druidly in any other games I play always interests me. And Necros are cool too!