So on the back of WotC's article today on The Future Of Masters, I think a Tribal Masters is an inevitability, even if it's not for a few years.
I think the trick to do it is to make lots of tribes that are spread broadly across colors, so you're not locked into a pre-built, linear deck as soon as you pick a color. Having an intersection of race and class tribes would also be key, like WotC touched on in Lorwyn block, and I wish they'd gone on to do more with.
Here are some tribes I think are likely, due to being spread across multiple colors and having a depth of support in the reprint pool:
Races
Merfolk: UGW
Goblins: RBG
Vampires: WBR
Zombies: BUW
Treefolk: GWB
Humans: WGX, White and Green have the most synergy but more Humans could be added in any color
Spirits: WUX, White and Blue have the most synergy but more Spirits could be added in any other color, especially with Kamigawa block cards
Elementals: RUGX, could add more for W or B
Elves: GB, not as broad as other colors but hard to see a Tribal Masters set without elves
Slivers: WUBRG
Beasts: GRX
Dinosaurs: GRW, a new tribe, but a popular one.
Classes
Wizards: UBR
Pirates: UBR
Rogues: UBR (man, Grixis has a lot of strong class tribes)
Shaman: GRB
Warrior: BRGW
Soldier: WRUX: Traditionally White, but quite a few in Red and Blue and even some in Black and Green.
Cleric: WB, also a smattering in other colors
Monk: GWRU
I can see tribal as theme but commander masters is more likely particularly due to whatever is happening with ezuri in that duel decks a way to get a bunch of highly sought after legends to have whatever is happening
Bad news is you forgot one tribe
C Eldrazi
You know there's eldrazi tribal stuff.
Oh and spirits is WUBG but WUBRG is preferred since there are red spirits that are good for spirit tribal I know since I play it
I think the trick to do it is to make lots of tribes that are spread broadly across colors, so you're not locked into a pre-built, linear deck as soon as you pick a color. Having an intersection of race and class tribes would also be key, like WotC touched on in Lorwyn block, and I wish they'd gone on to do more with.
Also, plenty of "build your own Tribal" ala Distant Melody, Shared Triumph, Cavern of Souls, the Kindred cards from Commander 2017, and others.
Here are some tribes I think are likely, due to being spread across multiple colors and having a depth of support in the reprint pool:
Races
Merfolk: UGW
Goblins: RBG
Vampires: WBR
Zombies: BUW
Treefolk: GWB
Humans: WGX, White and Green have the most synergy but more Humans could be added in any color
Spirits: WUX, White and Blue have the most synergy but more Spirits could be added in any other color, especially with Kamigawa block cards
Elementals: RUGX, could add more for W or B
Elves: GB, not as broad as other colors but hard to see a Tribal Masters set without elves
Slivers: WUBRG
Beasts: GRX
Dinosaurs: GRW, a new tribe, but a popular one.
Classes
Wizards: UBR
Pirates: UBR
Rogues: UBR (man, Grixis has a lot of strong class tribes)
Shaman: GRB
Warrior: BRGW
Soldier: WRUX: Traditionally White, but quite a few in Red and Blue and even some in Black and Green.
Cleric: WB, also a smattering in other colors
Monk: GWRU
Corrupt Control B | Burn R | UG Turbofog UG | White Weenie W | GW Tethmos WG | BG Cycling Combo BG
Enchantress GBW | Colorless Tron C | Red Deck Wins R | UG Madness UG | Mono-G Tron G | UR Puzzlehorns UR
Rhystic Tron WU| WU Prowess WU | BR Reanimator BR | Mono-R Control R | Stompy G | Temur Tron URG
Mardu Infinite Priest WBR | 85-Card Dredge BRG | Elves GU | Boros Bully RW | Jeskai Familiars RWU
Bad news is you forgot one tribe
C Eldrazi
You know there's eldrazi tribal stuff.
Oh and spirits is WUBG but WUBRG is preferred since there are red spirits that are good for spirit tribal I know since I play it
Oh and dinosaurs are confirmed Deciduous so it may happen
http://markrosewater.tumblr.com/post/169636717328/are-we-going-to-get-dinosaur-printings-in-future