MTG Salvation

For a possible (and likely 3rd return to Ravnica)the masterpieces in the set would need to reflect the theme of the set. For BoZ it was lands and Kaladesh it was artifacts. It's fairly obvious the theme of Ravnica are the guilds or mechanically speaking: two-colored gold cards.

That's why it is quite likely the masterpieces of Ravnica will be two-colored gold cards. Given that there are 10 two-color combinations I would estimate about 5 cards per combination for a total of 50 masterpiece cards. These masterpiece cards (let's call them something like Exemplars or Guild Paragons) will have a unique border but it would be really cool if each guild had a specific border for each.

Below I have compiled a list of 4 possible reprints for each color combination. Why 4? Well the Gearhulks have set a precedent that masterpiece cards can include new cards printed in the very same set. It would not be unlikely that the 5th card for each combination will appear in the 3rd Ravnica set. For my list I did not include planeswalkers as well as legendary creatures or spells that are definitely not from Ravnica like Atarka's Command or Oona, Queen of the Fae. I based this list on the need for a reprint and a personal wish for a Ravnican re-imagining of certain off-world cards like Bloodbraid Elf and Kitchen Finks.


GW:
Kitchen Finks
Knight of the Reliquary
Voice of Resurgence
Fracturing Gust

RG:
Dragon Broodmother
Manamorphose
Boartusk Liege
Bloodbraid Elf

BR
Fulminator Mage
Rakdos the Defiler
Dreadbore
Blightning

UB
Baleful Strix
Mind Funeral
Nemesis of Reason
Shadow of Doubt

WU
Grand Arbiter Augustin IV
Meddling Mage
Supreme Verdict
Absorb

WB
Deathbringer Liege
Debtors' Knell
Orzhov Pontiff
Vindicate

WR
Lightning Helix
Waves of Aggression
Boros Charm
Aurelia, the Warleader

UR
Electrolyze
Nin, the Pain Artist
Dominus of Fealty
Stitch in Time

UG
Fable of Wolf and Owl
Voidslime
Experiment Kraj
Progenitor Mimic

BG
Abrupt Decay
Lord of Extinction
Squandered Resources
Deathrite Shaman

Some of these seem likely, but there are a couple of things to remember:
1- None of the Kaladesh Inventions are cards that would be Common or Uncommon by current standards.
This rules out cards like Boros Charm, Blightning, etc.

2- There are only so many cards people want to ultra-bling, and Legendary Creatures are one of the main categories.
We can probably expect Ravnica Masterpieces to be weighted heavily in favor of Legendary Creatures.

Ravnica doesn't have ouphes, so no Kitchen Finks, Dragons are largely extinct there as well, except for Niv's breed, so no Broodmother either. It's not clear Nin is from Ravnica, and there are possible indications against it, such as not having the mutant type while being a 4 armed Vedalken, when normal Vedalken on Ravnica have 2 arms, and her staff first appearing in M13, which opens up the possibility she is from Shandalar (since some, but not all, new cards introduced in the M-sets are from Shandalar, and Shandalar can have any sort of races or whatnot due to the portals and stuff that goes on with that Wandering plane, if I recall correctly). Also, I'm pretty sure we don't get dead stuff in masterpieces (unless they are undead and still around, if you know what I mean), they aren't free from the timeline, so Experiment Kraj might be out of the running as well. Supreme Verdict is also a potential no-no, given the story stuff involved in it if you read the e-books they put out for RTR block, it isn't something that should be actively in use, and probably hasn't actually been used.

As mentioned by Empathogen, you've got some issues with rarity as well.

I could actually see a new Ravnica block being a time when they might print a lot of old popular versions of planeswalkers as masterpieces, perhaps having it involve some conflict involving the gatewatch vs. enemy planeswalkers, but have planeswalker cards that couldn't be reprinted in Standard for balance reasons, such as Jace, the Mind Sculptor and Liliana of the Veil, given that the Gatewatch has decided to base themselves out of Ravnica seemingly. I do think there will be a lot of multi-color cards as well, since there aren't enough planeswalkers that would likely be there and have such popular versions to do things that way. Cards I could easily see as being included include:

Sphinx's Revelation
Voice of Resurgence
Progenitor Mimic
Abrupt Decay
Loxodon Smiter
Deathrite Shaman
Glimpse the Unthinkable
Privileged Position
Slaughter Games
Shadow of Doubt
Orzhov Pontiff
Debtors' Knell
Voidslime
Meddling Mage
Absorb
Angel of Despair
Waves of Aggression
Fracturing Gust
Wheel of Sun and Moon (edit: or maybe not, not sure if Ravnica even has a moon, or if it might have more than one moon)
Memory Plunder
Undermine
Defiler of Souls
Everlasting Torment
Decimate
Vexing Shusher

I would just like to say that having the Ravnican masterpieces be powerful dual color cards from throughout magic's history, with each masterpiece having a special frame to correspond to the guild it's colors match up with would be amazing.

Example: Art of a Rix Maadi mage obliterating someone with Terminate with a frame that looks like it's made of shoddy dark iron splattered with blood with a Rakdos guild insignia somewhere on the card.

I like this, and it makes a lot of sense. Maybe with some of the better guild champions thrown in as well (the good ones - Agrus Kos isn't the same power level as original Teysa)