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
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
If it was a circle then the card was common
You return a creature card from your graveyard to your hand, draw three cards, then gain 5 life
Whenever Mirko Vosk deals combat damage to a player, they have to reveal cards from the top of their library until theyve revealed four Land cards