Mage in RoS is in an awkward position. The class has developed a few solid archetypes, but all of them revolve around the insanely overpowered Conjurer's Calling/Mountain Giant interaction. This is my attempt to climb the ladder with Mage without resorting to using either of those cards, and it's been surprisingly successful so far. Ironically, the best matchup for this deck is other Mages, but its performance is serviceable against most decks regardless.
You want to mulligan for your early spell damage cards (Spellshifter, Spellzerker, and Bloodmage Thalnos) and your cheap minions which synergize with them (Spellbook Binder and Arcane Watcher). Against aggressive decks, you definitely want cards which can go wide like Cinderstorm, Blast Wave, and, most importantly, Shooting Star.
A few card suggestions below:
Spellzerker: This is one of the strongest cards in your deck. Coining it out on Turn 1 is huge and I almost always do it if given the opportunity. Don't be afraid to use your hero power to damage it; the spell damage boost is often worth it.
Stargazer Luna: Removing a 4 health minion on turn 3/4 is not that difficult for your opponent to do and therefore I would never play this card on curve unless you are in a situation where you might be able to get away with it (i.e you're playing Bomb Hunter and he has no mechs on the field). Still, it's usually best used in the lategame to turn your cards into cantrips.
Scorch: I was initially skeptical of including this card but it performs surprisingly well. You have 6 elementals in the deck, so keep this card in mind while deciding whether to play them. You can think of this card as a substitute for Spellbreaker since it can cheaply remove minions you would normally silence (a Venomized Missile Launcher or a Zilliax'd mech, for instance).
Azerite Elemental: This card gains Spell Damage +2 every single turn it is alive, so it can blossom out of control if your opponent cannot remove it promptly. It's a good card to play on curve, unlike its fellow elemental Cosmic Anomaly, but it's important not to get too greedy and wait for it too long before playing your spells.
Unexpected Results: The buffs certainly gave this card the shot in the arm it needs. It's still not very impressive on its own, but it improves exponentially with spell damage (if you can get the cost up to 6, that right there can be a game-winning turn). In general, I usually wouldn't play this card without spell damage, but if you're desperate for spell damage minions, there's a non-trivial chance you can get one in the 2-cost pile.