While my Mage is consistently Frost, I'm going with Arcane because of the change to its Mastery. Arcane has, since Cataclysm introduced the stat, really revolved around its Mastery, which until Legion will have increased damage done based on the percentage of your max mana you currently have.
In Legion, the Mastery will instead boost your max mana and mana regeneration as well as the bonus damage of your Arcane Charges (which will now be a secondary resource.) Thus, your damage will not depend on how much mana you currently have, but more on how much you're willing to spend. The danger, then, will not be of reducing your mastery bonus by too much and thus doing sub-par damage. Instead, the danger will be of not having the Mana to actually cast your spells. In a lot of ways, this mirrors how things were in Wrath of the Lich King. You will be able to have very powerful sustained burst (if that makes sense) but you'll pay the price for it by having to play conservatively for longer to get back your Mana.
As always, this is Alpha, so remember that both numbers tuning and playstyle design can and likely will change before this goes live. Let's get to the abilities!
Mastery: Savant will increase your Mana and your Mana Regeneration rate by equal percentages, while increasing the damage bonus of your Arcane Charges by a different percentage.
Arcane Blast is your main spell, with a 2.25 second cast time, it deals Arcane damage and generates an Arcane Charge. For each charge, the damage is increased by 50% (before mastery) and the cost is increased by 100%.
Arcane Barrage is instant, on a 3 second cooldown and consumes all your Arcane Charges. It increases in damage by 50% for each Arcane Charge and hits one additional target per Arcane Charge for 50% of its damage (on each extra target.)
Arcane Missiles has a chance to be activated by other damaging spells. It is channeled over 2 seconds, launching 5 waves of missiles over that time. It deals 50% additional damage per Arcane Charge and generates an Arcane Charge.
Arcane Explosion is an instant-cast spell with no cooldown, dealing damage to all enemies within 10 yards and refreshes your Arcane Charge duration as well as having a 30% chance to generate an additional one.
Arcane Power is a 1.5-minute cooldown lasting 15 seconds, increasing damage by 20% and increasing Mana costs by 10%.
You get the idea.
If you've played an Arcane Mage any time in the last few expansions, this should be familiar as more or less exactly how it has played before. The real change is with the Mastery. There could also be some interesting alterations thanks to talents and artifact traits.
Really I think what makes this Mastery change interesting is that it gives you far more flexibility - you can use your mana bar as you see fit, instead of always trying to keep it as near to capped as possible. If this design remains, you can expect to see Arcane Mages doing insane dps in short fights, as they'll be able to spam Arcane Blast.