I would! I do think the schools are deep enough now that we don't NEED new spells. A full Arena expansion like paladin vs siren is a huge amount of work, which is why we might not see a new one again. But a new mage card can really open up whole new styles of play.
Perhaps they could make Arena packs similar to Academy expansions. For example, the female wizard with perhaps 6-12 (24 to 60ish cards) new spells that tie in to her abilities. There are several promo cards that can be slotted in as well.
Here's my top wish list
Arena Elementalist: The academy version is awesome, can actually make academy interesting to play. I would love to see a few more frost spells and an arena version.
Alt Wizard: It would be nice have another option for arcane. I'm sure some are a bit scarred/sick of the wizard.
Alt Druid: Either a port of the academy or a new alt druid.
Alt Forcemaster: I guess. For those players
Alt Paladin: Huh, I didn't even think of this one! Now I'm kinda excited by it, more Valor!
Anyway I think individual small arena packs for each mage might be more realistic. But if they did put all the mage cards in one last pack I would buy it up in a heartbeat.