You are almost certainly right that the stars in a High Fantasy world would be different than our own, but there is almost nothing in the world that I'm less interested in than Astrology, so of all the things in the world that I would want to build an intricate novel system for, a unique take on Astrology ranks near the bottom.
I am entirely happy to use a straight pastiche of real world Astrology for any astrological needs I would have in my fantasy world.
That said, it would require very little effort to replace existing astrological signs with counterparts on a one to one basis.
The standard zodiac signs have elemental correspondences, including what we would call para-elements. So you have a sign for pure Fire, earthy Fire (lava), and airy Fire (smoke). Each of the signs also corresponds to a specific body in the solar system, including the Sun, Moon, and each planet. Each of these celestial bodies is also ruled by a specific god. We can find monsters that are symbolically similar to the standard zodiac creatures, relate them to specific gods, and add the energy and quasi-elements...
It's been done. You want Dragon issue 340, as that had astrology as it's theme, including a zodiac composed of beholder, harpy, ettin, dryad, stirge, wyvern, dragon, unicorn, hydra, chimera, kraken and basilisk, along with mechanical support for people born under each sign and appropriate spells & magic items.
My 4e Star Pact Warlock was astrology / astronomy -themed, not an insane Far Realms devotee. From my notes and off the top of my head:
- Leo the Lion is, in one culture, Leonard the Farmer. He carries a sickle in one hand and guides a plow in the other. When the plow rises at dawn it is time to sow the seeds for a crop. When the sickle sets at dusk it is time to reap.
- Four constellations could be Father, Mother, Daughter, Son -or- King, Queen, Prince, Princess.
- One star is the most malefic sign of portending DOOM. It might be the Disintegrate eye-ray of the Beholder.
- Every culture names the constellation we know as Orion after their mythic hero-founder character.
- Argo the great Ship, which explores new lands, crosses over the horizon, "boldly goes where no man has gone before." After a great cataclysm when the gods came down to walk the earth, the stars of Argo twisted in the sky and became the Shipwreck.
- The stars of the Peacock are visibly different colors - blue, red, yellow, orange, white. The stars of the nearby Cockatrice all look dull gray, like rock.
- One constellation is called the Triangle by barbarian cultures, the T-Square by those cultures who build monument buildings, and the Geometer by Wizard enclaves and other learned bodies, because faint stars nearby form squares and demonstrate the equation a^2 + b^2 = c^2.
- If you understand how to read the signs in the area of the sky called The Sea (several bordering constellations have something to do with sea creatures), you can find Atlantis!