I've had similar thoughts to this when I was trying to design a fair pauper dual land that's good for aggro strategies but wouldn't turn the format into rainbow colored good-stuff piles. One idea was to make it based on the guild colors or add land count restrictions similar to fast lands
Land
Common
~ enters the battlefield tapped.
When ~ enters the battlefield, add one mana of any color to your mana pool if you control two or fewer lands.
T: W
Land
Common
~ enters the battlefield tapped.
When ~ enters the battlefield, add one mana of any color to your mana pool if you control two or fewer lands.
T: W