And I don't see what's going on with your math. I'll assume a big set and ignore the possibility of foils or masterpieces. You have a 1/5 chance of getting a particular basic land from a booster, or 1/20 if you're looking at different art as different cards. You have about a 1/10 chance of getting a particular common, about a 1/27 chance of getting a particular uncommon, a 1/60.5 chance of getting a particular rare and a 1/121 chance of getting a particular mythic. So they're either more common than commons or less common than commons but still more common than uncommons, depending on how you treat the different art.

You're doing a PARTICULAR card.
My math is ANY card of a given rarity.
Each booster pack contains 1 basic land and 1 rare or mythic.
Also, while you have a 1/5 chance of getting a particular TYPE of land, you generally have a 1/20 chance of getting a particular CARD (each land has 4 different printing).

Er, I already brought up the 1/5 vs. 1/20 thing. Did you not read my post?

I now see that math you're using, but it doesn't make any sense to use that particular metric. Cards aren't considered "common" because there are a lot of them in the pack, but because the combination of how many there are in a pack and how many different ones in the set will mean that each particular one is pretty likely, and you'll start getting duplicates of those earlier and more frequently than the other rarities.

To illustrate why your metric isn't useful, suppose they made a hypothetical set with only a single mythic (and no rares) but 10,000 different commons. That mythic would appear in every booster, so even if it's the most useful card in the set, it would quickly become worthless, as everyone would already have all the copies they need. Meanwhile, the price of the best commons would be astronomical, since there would be so few of them in existence.

Or just suppose that in one set, the whole printing and collating system was exactly the same as normal, but for a single common in the set, they gave it a special expansion symbol color. That card would still be just as likely as any other common, but by your metric, we'd now have a special rarity appearing only about every ten boosters, making it rarer than mythics.

I honestly think that, at this point in time, changing the rarity color of basic lands would cause more harm than is worth it. Because you can't go back and change the 23 years worth of basic lands so that they have the new rarity color. So new players who have no idea about older sets facing off against older players might start some argument about how "That basic land is fake! It has the wrong rarity color!"

In 2008, you could have said the same thing about Mythic rare.

In 2006, you could have said the same thing about special.

You're suggesting a change for all sets moving forward that affects cards that have been in Magic since the very beginning of the game.

Strictly speaking, the Masterpiece/expeditions do get their own symbol, it's just not differently colored, but it IS a different symbol.

Barinellos was saying that it isn't differently colored, but that the symbol itself is different (i.e. there's an expansion symbol exclusive to "Zendikar Expeditions" and one exclusive to "Kaladesh Masterpieces").

I don't have access to any recent physical cards, so I can't compare those. But from the images online, the expeditions and masterpieces just look to have the same orange as mythics. The actual cards would also have the foiling that could distort that color.