In practical terms you can replicate most of Blame! by replacing The City with "a random Hive in the Imperium"
I think there are lots of things you can do with the C'tan, starting at a baseline of "their original concept, just executed slightly better and with more material". My personal preference takes inspiration from the old implication that the Outsider was trapped in a Dyson Sphere; what if "feeds on stars and worlds" was just a typically poetic Aeldari description of a vast machine powered by stars or molten planetary cores? The C'tan are literal Machine Gods – colossal, reality-warping intelligences akin to SHODAN, AM, the Inhibitors, Cynosure, N2, the WAU, the Anti-Xeelee, the IDE, or a Culture Mind, constructed around stars or within the cores of Tomb Worlds, created to direct Necrontyr society with inscrutable calculations and godlike intellect. Replace the "shards" or "sheathes" with remote avatars, constructed from necrodermis and hard light.
The heart of a C'tan is a point where space-time warps under the impossible density of electric thought, where the solar visions of a god that was never born are carried down crystal channels laid by machines who could understand the machines who could understand the machines who could understand the machines who could understand eternity. Where time stops and space makes no sense, a labyrinth of quantum mechanics, an indecipherable autoconstructing mind of steel and starlight, ever-dreaming its way into ever-deeper depths of impenetrable cognition, its logic as dark and inescapable as a black hole. The Lords of the Necrontyr were the high priests of this unfathomable Deep Science, administrators and favoured agents of the machines that made them. The C'tan guided Necrontyr society, engineered its evolution over generations of alien efficiency. The Necrontyr were weighed down by their bodies, held back by their minds. The Necrontyr died, and the Necrons were born. It was intelligent design.
Perhaps millennia of degradation and decay, forced into dormancy as wild warp-storms interfered with their relentlessly logical calculations, have driven the C'tan mad. But how could you tell? They think on a scale too massive and complex for lesser minds to comprehend. They can only devote themselves to its impossible insight, or perish. Perhaps Mars is one, and the ghosts of its rusted machine-mind still haunt its depths. Perhaps it was called The Dragon. Perhaps its dark data dreams laid the foundation for human science, and took true root in the souls of the first colonists of the Red Planet.
Now they turn in their sleep, now they
wake, sluggish and fitful, and their immortal agents move to interpret the will of their mad gods. The Keeper Of Infinity, who gathers countless specimens to further his Tomb World's understanding of this squirming new galaxy, harvesting whole Hives and seeking out specific samples to sieve the human genome for the Old Ones' secrets. The Storm Lord, who directs cybernetic legions with the uncanny swiftness and precision of a chess master, guided by the thundering god-neurons of his C'tan's atmospheric emanations. The White Despot, whose ships whip through deep space with a silent scream, endlessly hunting for Aeldari Craftworlds, seeking to end a war that was won aeons ago. Sorrow In Runes, scion and architect of The Outsider, the faceless spider-priest whose artistry wields asteroids as a lesser artist uses chalk, reshaping systems into more orderly forms. Tools of a grand cleansing. Heralds of a Clockwork Cosmos. Deranged relics of broken machines.
