By moving on from the current system, we can pay homage to Ethereum’s history, while working toward something unique that can be long-standing, and that meets the goal of representing both layers as “parts of a whole”.
On the first note, the original Ethereum roadmap included the four stages: Frontier, Homestead, Metropolis and Serenity.
Frontier and Homestead were delivered, as was Metropolis, albeit in three stages (Byzantium, Constantinople, Istanbul). The final stages of that original vision included a complete proof-of-stake and sharding. Relatedly, EIP-4844 marks the start of sharding, after Shanghai closes out the PoS upgrade and withdrawals. This makes for a great opportunity to finally close that original chapter.
I’d first propose that the name of Serenity be applied to the EIP-4844 upgrade, marking an end to the celestial naming scheme that’s touched both the original roadmap and the current CL forks, before moving forward to a greener future.
For the future naming scheme proposal, an initial idea was to choose something that reflected the CL/EL, while having a parent-name that solidifies the relationship between two parts of one whole. For example, if one upgrade package was dubbed “Purple”, upgrades within it would consist of two sides, with one layer designated as “Blue” and the other “Red”.
There were a number of potential ideas discussed here (stars, sister cities, rivers), some of which I’ll include below. But these days, one of Ethereum’s unique powers is its ability to continue to develop independently of any original plans and roadmaps, founders, or a few OGs. This natural growth (combined with “garden” mantras that have taken root and the PoS narrative meshing well with a greener/cleaner system) led to exploration of botanical or ecological schemes.
Specifically, I’d propose to make use of families of trees and species within those families to accomplish many of the goals set out above (reflecting on life, ecology, evolution, strength…). There are enough unique families and varieties to apply to upgrades for years or decades to come. Examples, like Fir and Cedar as two sides of a package dubbed “Pinaceae” would do well to convey this natural relationship in a unique way. There’s enough diversity to (a) move in alphabetical order and/or to (b) skip specific species if/when there’s something too odd to include a name, or to (c) include more than two names for upgrades that require multiple steps. Some even come with their own (controversy-free) emojis 
! And on ordering, Latin names can be used as something more neutral if/when English names are less of a fit.
Tl;dr suggestion: After Shanghai, sharding begins with the Serenity upgrade, which pays homage to Ethereum’s history (past and present) and closes an important chapter before moving forward. Going into the future thereafter with something that reflects biodiversity, evolution and connectedness, like tree-species and families, encompasses Ethereum’s natural growth (merkle trees anyone?), uniqueness, diversity, and the green future created by the work put into bringing the Beacon Chain and Merge to life.
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JS
Initial ideations:
A. Binary Star systems.
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Package: System
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EL: Star A
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CL: Star B
Reasoning against: Celestial names have been used for the CL and by many Cosmos and other chains. We can do more to be original and aligned here.
B. Major Rivers/Cities
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Package: Major River (e.g. Nile, Rhine, or Tigris)
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EL: City A (e.g. Cairo, Bonn, Baghdad)
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CL: City B (e.g. Giza, Cologne, Mosul)
Reasoning against: While this version (suggested by Peri) is my favorite alternative to the above, this suggestion also gets caught in our current city-scheme that’s been used for the better part of a decade. There’s been pushback against flags and nationalism related to upgrades as well.
C. Planets & Largest Moons
Discussed and suggested by Tim above, this is solid and unique with many options, but continues the celestial track.
Thanks to Tim, Pari, Peter and others for their input!!
: ! Two years after your launch, you’re now successfully serving as the consensus layer for the entire Ethereum network 
!
if that’s not a concern, then single names work for me just as well 
).
Shapella (Shanghai + Capella)
Canubhe (Cancun + Duhbe)
Practra (Prague + Electra)
( for