Fiery Impulse
 
In my opinion, spell mastery is a perfect sort of ability word for a core set. It's simple, it's intuitive, it directs readers towards the sorts of decks it likes to be in . . . and, unfortunately, there are undoubtedly those among the online peanut gallery who will claim it's not flashy enough to be in a large expansion. They'd be wrong. Consider Fiery Impulse here, and how a lot of decks won't even notice its being less versatile than Shock, because they want to aim burn spells at creatures in the early game anyway and/or don't care to burn their opponent down in the late game (Spellheart Chimera and Sphinx of Magosi being as efficient and dominating as they are). It won't replace Lightning Bolt in older formats, but that's hardly the yardstick for direct damage spells: there are few casual decks that would be ashamed to use this card.
 
Constructed: 3/5
Casual: 3/5
Limited: 3/5
Multiplayer: 3/5
EDH/Commander: 3/5