You nailed it, mostly I expect this/these would show up in a format that has lots of colorless lands or wants to encourage monocolor. I kind of worry that this would make it too easy to cheese your way into monocolor though. Maybe it'd actually be a better effect on an Elf.
Seed Spreader1G
Creature - Elf Druid (U)
Tap an untapped land you control: Add G to your mana pool.
2/2
Another Twist
Land (R)
Tap two untapped lands you control: Add one mana of any color to your mana pool.
Just saying, Seedwind Vale is strictly better than a basic land, which isn't allowed. As far as I can tell, there's no reason not to run it in a deck containing green. So currently they shouldn't even be printed as rares.
That being said, I can see this effect being uncommon, especially once they have some form of downside attached. Maybe only letting you tap other lands? That way it would encourage playing multiples of these for colour-heavy decks, and monocolour couldn't just splash 4 of these to make their Nykthos, Shrine to Nyx or [insert other colorless land here] stronger when played early.
Edit: And remember, Urborg gives the effect to your opponent as well, and is legendary. It costs around $10. Your land does neither, making it close to strictly better.
Seedwind Vale
Land (?)
Tap an untapped land you control: Add G to your mana pool.
Seed Spreader 1G
Creature - Elf Druid (U)
Tap an untapped land you control: Add G to your mana pool.
2/2
Another Twist
Land (R)
Tap two untapped lands you control: Add one mana of any color to your mana pool.
That being said, I can see this effect being uncommon, especially once they have some form of downside attached. Maybe only letting you tap other lands? That way it would encourage playing multiples of these for colour-heavy decks, and monocolour couldn't just splash 4 of these to make their Nykthos, Shrine to Nyx or [insert other colorless land here] stronger when played early.
Edit: And remember, Urborg gives the effect to your opponent as well, and is legendary. It costs around $10. Your land does neither, making it close to strictly better.
Avant Block: Avant -- Stormfront
Other than that, this effect is justifiable at rare or uncommon. Probably rare unless your limited environment needs it.