Lorwyn featured dual lands for 5 of the 10 color pairs (Gilt-Leaf Palace, et al.) Trying to think of ideas for rest of the pairs, but I'm running out of good ideas for tribes in the color pairs. The Lorwyn lands were for Elf, Goblin, Giant, Faerie, and Merfolk, and there were similar but varied lands for Treefolk, Kithkin, and Elementals in Morningtide.
Profane Manor
Land
As Profane Manor enters the battlefield, you may reveal an Vampire card from your hand. If you don't, Profane Manor enters the battlefield tapped.
Tap: Add W or B.
Mage's Tower
Land
As Mage's Tower enters the battlefield, you may reveal an Wizard card from your hand. If you don't, Mage's Tower enters the battlefield tapped.
Pride Lands
Land
As Pride Lands enters the battlefield, you may reveal an Cat card from your hand. If you don't, Pride Lands enters the battlefield tapped. T: Add G or W.
Not a very common tribe but the most distinct GW tribe as far as I can tell. For RG there are many options but I think it has to be Dinosaurs. Elementals are too spread among all colors, Beasts and Kavu are outdated, Warriors and Shamans never got any tribal support.
Lost Valley
Land
As Lost Valley enters the battlefield, you may reveal an Dinosaur card from your hand. If you don't, Lost Valley enters the battlefield tapped. T: Add R or G.
Merfolks are very solidly in green now. It's not only a Ixalan thing, Return to Ravnica had establish the connection between green and merfolks in simic. Notice the first merfolk to be playable in mono-green (Merfolk of the Depths) is from that block.
Coral Acropolis
Land
As Coral Acropolis enters the battlefield, you may reveal an Merfolk card from your hand. If you don't, Coral Acropolis enters the battlefield tapped. T: Add G or U.
Now we need a new WU one. It should be either Spirits or Birds. Spirits is by far the most supported one but I'm a bird guy, what can I say ?
Hovering Aery
Land
As Hovering Aery enters the battlefield, you may reveal a Bird card from your hand. If you don't, Hovering Aery enters the battlefield tapped. T: Add W or U.
Blue/Green or Blue/White merfolk are more a function of the standard environment than them being religated to that color pair, but the land already exists for Blue/White so I'd rather find a different option for U/G. I really like the Dinosaur idea, and I can't think of something much better than Cats for G/W.
If I were going to reassign pairs, though, I like your ideas for Birds or Spirits both
Profane Manor
Land
As Profane Manor enters the battlefield, you may reveal an Vampire card from your hand. If you don't, Profane Manor enters the battlefield tapped.
Tap: Add W or B.
Mage's Tower
Land
As Mage's Tower enters the battlefield, you may reveal an Wizard card from your hand. If you don't, Mage's Tower enters the battlefield tapped.
Tap: Add U or R.
G/W - Druid?
R/G - Warrior? Shaman?
G/U - ???
Land
As Pride Lands enters the battlefield, you may reveal an Cat card from your hand. If you don't, Pride Lands enters the battlefield tapped.
T: Add G or W.
Not a very common tribe but the most distinct GW tribe as far as I can tell. For RG there are many options but I think it has to be Dinosaurs. Elementals are too spread among all colors, Beasts and Kavu are outdated, Warriors and Shamans never got any tribal support.
Lost Valley
Land
As Lost Valley enters the battlefield, you may reveal an Dinosaur card from your hand. If you don't, Lost Valley enters the battlefield tapped.
T: Add R or G.
Merfolks are very solidly in green now. It's not only a Ixalan thing, Return to Ravnica had establish the connection between green and merfolks in simic. Notice the first merfolk to be playable in mono-green (Merfolk of the Depths) is from that block.
Coral Acropolis
Land
As Coral Acropolis enters the battlefield, you may reveal an Merfolk card from your hand. If you don't, Coral Acropolis enters the battlefield tapped.
T: Add G or U.
Now we need a new WU one. It should be either Spirits or Birds. Spirits is by far the most supported one but I'm a bird guy, what can I say ?
Hovering Aery
Land
As Hovering Aery enters the battlefield, you may reveal a Bird card from your hand. If you don't, Hovering Aery enters the battlefield tapped.
T: Add W or U.
BGU Control
R Aggro
Standard - For Fun
BG Auras
If I were going to reassign pairs, though, I like your ideas for Birds or Spirits both