Took some time until a draconic theme was suggested.

What I always wanted to see in an expansion is not simply a draconic humanoid mage, like the Dragon Lord/Rider. What would be AWESOME is the Dragon himself!
No need to call him/her Dragon Mage, dragons are powerful magical creatures and spell casters by nature. Here are some ideas:

Dragon: probably a young dragon, curious about the humanoid world and to learn if what they call magic in the famous arena is something of promise or just amateur tricks of the fledging races that still have much to learn about what yielding magic is really like.

There are a lot of different ways to develop this concept (and I can suggest a few of them ranging from very traditional to more radical approaches), but they would have to attend some iconic items:

Breath Weapon: this would be the mage's power. Basically it's a powerful attack of a given element (or arcane/force), which would also be a favored school for the dragon. It could not cost much mana, but need to recharge for a few rounds before being usable again.

No or restricted equipment: dragons usually don't need or care to wear armor, clothes etc, they don't need weapons beside their teeth, claws and other natural weaponry. Restricted equipment would account for one development concept in which the dragon summons part of his hoard to the arena (or connect with it through an apparatus) in order to empower himself, possibly also weaving time forward, to grow bigger, older and deadlier. This would result in thougher scales, wing attacks, increased damage and the like.

Servants: although it's not mandatory, dragons have an easy(ier) time acquiring servants than most creatures, so they would be able to count with several underling cards, specially draconic ones like different kobold cards, draconic versions of some already existing creature cards, lesser dragons such as wyverns. I think it's fitting that draconic creatures like those are of the arcane type.

Battle prowess: dragons are notoriously a superior race and this also mean they are powerful melee fighters (who can fly)! Specially in a no equipment development path, dragons would have to have some armor, more life (perhaps), flight, multiple attacks and special melee damage. One way to represent this in a balanced manner is to make the dragon count as a already buffed/equipped mage and for that reason, greatly diminish the size of his spell book.

Of course there's a lot more to think and discuss, but the intent is to present a general idea to contribute to this great game and this cool community! Please, feel free to comment and shine your own perspective on this larger than life mage idea and game design challenge!