If I recall correctly, this began in the core set with Garruk, Primal Hunter with a rough five-cost with triple Green, but then +1 to make a 3/3 Beast that's Green or -3 once to draw
I'm glad now that I have some distance from my job because I've realized that an important part of what I do (as a creative endeavor) is allow things to percolate
In general, I would look at these two factors when looking to include cards not of your tribe's creature type: Is this creature so strong that it, on its own, is generally stronger than my tribal creatures when they are receiving bonuses and/or empowering my other cards
But the reason Aura Sphere stands out is the ability to put pressure on the bench by dealing 30 additional damage to any target card sitting at the back
Atog Atog | Illustration by Jesper Myrfors This one is for my son, who avoided saying yes to learning Magic with me for a year when he was younger because he saw this red card, recoiled, and peaced out