WARNING: a rant on Colorless mana , custom cards, flavor discussions, possibility of C outside of this set adn whatever else cross my drug-filled mind
Honestly i don't think colorless mana should be linked to eldrazi flavor only. It could be linked in general to "otherworldy" flavor, where some spells are colored but also have kind of special attributes that link them to colorless mana. We've used colorless mana for 25 years and no one ever told us it was special. Now that it is in mana costs, i don't see it at particularly controversial if it was treated as just another way to cost things with a more refined approach. Like GC is harder to cast than GG, GX or 1G. It can also express a sort of purity of color of the spell without having to cost like GGGG.
Also if i'm not mistaken Ugin has been defined in the lore as using colorless magic, but there are no cards tied to him with C in them so i'm a bit uncertain if he necessarily represent what C can do.
I think Theros with their Nyx theme would have worked perfectly for C +colored costs spells instead of a vague "enchantment theme", to indicate their otherwordly nature, and with its focus on the "purity" of mana (devotion could have counted C costs too or worked like "the number of colored permanents of the same color, permanents with also C in their costs count twice"). Theros scrylands would've been actually good (as in-playable in legacy/modern) if they gave C too. Also, Shadows over Innistrad with its "corrupted by Emrakul" theme in the next expansion could also give us some mixed colored-C cards, with emrakul slowly seeping in the world of innistrad represented by said cards. I imagine Emrakul's theme could be to be resilient to colored spells/effects, maybe self-color as to represent madness of self-hating after being touched by Emrakul's presence. It also differentiate it nicely from Kozilek (effects based on colorless mana), and Ulamog (exiling things and using exile as a resource)
As an example of what i mean on a imaginary SoI card:
Tormented Geist CW
Creature - Spirit
Flying, protection from white spells
2/2
Spirits and otheworldy creatures would be touched first by the influence of Emrakul , but some sentient evil creatures could volountarily search for its influence as a mean to gain power over worldy magic or simple curiosity mixed with arrogance, like vampires:
Mind-Torn Vampire 1CR
Creature - Vampire
Intimidate.
Colored spells that target this creature cost :2: more to play.
4/3
Artifacts and spell could easily be made researching Eldrazi magic by blue mages and artificers in general, like:
Search Beyond CU
Sorcery
Exile the top three cards of your library, then choose a exiled card you own and add it to your hand.
Eldritch Tome 2
Artifact
C, T: exile target card in a graveyard.
C,put a card you do not own from exile into their owner's graveyard, T: draw a card.
Literally power beyond your comphrension
As of now, i think those are the key identities of Colorless mana:
- usage of exile as a resource and exiling things both in play, hand or library. Exiling graves hasn't showed until now iirc and i like it as a sorta defining characteristic
- creation of drone/scions
- hate on colored things (emrakul, all is dust, Ugin)/affinity for non-colored things (kozilek and a tons of other cards) , or just hard to target with spells in general (reality smasher)
Less common but still seen:
- card draw (Sea-Gate ruins, both Kozileks, EndBringer) and creature P/T manipulation (Eldrazi Mimic, spatial contortion) or destruction based on P/T (warping wail)
- cheating cards into play (this was literally only on matter reshaper and it had more of a flavor of exchanging "forms").
Finally an idea for lands to fix this mana if it ever were to be a theme in a future set.
Desolated Chapel (cycle of Desolated X)
Land
T: add C to your mana pool
T: add W or B to your mana pool. Use this ability only if you control 3 or less permanents.
Yeah those are essentially tri-lands. It's a bit harder to desing C-colored lands since they'd be so close to actual basic lands in term of power level already. I definitely don't want those to be fetchable, fetches are already way too good in every format. Also as a final point, Wastes were printed with a-generically-as-possible name for a reason.