For about two thousand years the basic structure of astrology has been the same, there has been 12 signs of the Zodiac, and 7 planets. The earliest known depiction of the classical Zodiac is in the Hathor temple in Dendera, from around 50 B.C. and it is from the Ptolemaic period, so it is of Greek origin, but many of the signs can be found in earlier zodiacs from both Babylonia and Egypt. The Greeks constructed the system of the elements in the zodiac, and gave the planets ruler-ship over the signs, and developed the horoscope that makes use of the ascendant and the twelve houses. From there on, not much changed; though the Arabs made a few contributions.

This article is not about the history of astrology, but a speculative conclusion and suggestion about its development; and so, for a deeper understanding of the history of astrology, please consult other sources.
New Planets
When new planets suddenly were discovered, they brought with them complete changes to human consciousness and society. Perhaps it was that the change of consciousness first happened, making the discovery of the planet possible; or all was part of human spiritual evolution? However, that it would have been pure coincidence is repugnant to reason; it is also not in the spirit of astrology, which makes the assumption that there is something more to the Universe, a deeper meaning and awareness. Astrology requires faith to be understood, but no faith is needed to make correct interpretations through the faculty of reason alone.
Uranus
The planet Uranus is visible to human eyes, but was only recognised as a planet in 1783. This tells us a lot about astrology, in it's essence, that it concerns the consciousness of humanity, not the physical facts. Uranus have been there as a physical planet for billions of years, even visible, but because of its slow orbit generally was mistaken for a star. At about the same time the planet was recognised, something happened to human consciousness: the Enlightenment happened to humanity. This is the astrological meaning of Uranus, it is about greater knowledge and understanding, and freeing consciousness from dogma and superstition. During this period we see the emergence of the Industrial Revolution; development of science; the downfall of the Christian empire and monarchy, replaced by new political ideologies and philosophies.
Neptune
As with Uranus, Neptune is also visible to the naked eye from earth, and has therefore been seen in the night sky without having been recognised as a planet until it was discovered in 1846, and named after the Roman god of the sea. In the human consciousness an interest for spiritism, magic, nature and natural science, art and poetry occured; it was the rising of the Romantic era.
Pluto
The Romanticism lasted until about the time for the discovery of Pluto. What happened was the World War I, and death and destruction came into the human consciousness. With the invention of the machinery, death became an industry; and was capitalised. In 1930, a few years after the ending of WWI, Pluto was discovered and named after the Roman god of death and riches. After that followed WWII, and a series of wars that shook the earth, it was bloody, and it made manufacturers of weapons filthy rich. Never before had capitalism been as powerful; the world became a plutocracy—ruled by the richest.
How to kill the planet of death
Pluto was never killed, but it was demoted to a dwarf planet, and so its influence diminished from 2006. Incidentally the movie Zeitgeist was released in 2007, promoting a society without money at all, awakening the realisation that money isn't needed to have a functioning society; and in 2008 Bitcoin was invented to give the world a new currency outside of the plutocratic empire.
Minor Planets
Having seen a new planet every century for three hundred years, what happened by the end of the 20th century was quite amazing; suddenly a lot of smaller planets were discovered. In 2006, these planets were categorised as Dwarf Planets; and so we could say that the dawn of the 3rd millennium came with a lot of new planets, the preceding three centuries gave us three planets, and in the early millennia-shift one of them were demoted and a lot of new were discovered and classified as Dwarf Planets; these include Ceres, Eris, Haumea, Makemake and Sedna. Other minor planets were discovered and called Centaurs, among which is found Chiron. What does that mean? Well, human consciousness is no longer monolithic, human society is no longer centralised. everything is multitude, and much happens at once. There is no longer one culture, but we talk about multi-culture; this can also be seen in the fact that planets are no longer named after the gods of the Roman Empire.
New Signs, a New Age
Previously, only new planets were recognised. No major changes were made to astrology, but three signs were given new rulers: Aquarius were given Uranus as a ruler; and Pisces were given Neptune; to Scorpio were given Pluto. At the same time, the old rulers remained; thus Aquarius is ruled by both Uranus and Saturn. Changes to astrology must keep it backwards compatible, because otherwise it would make earlier horoscopes invalid. Additions must be like layers placed on top of previous versions of astrology. Therefore we can add new rulers to the signs of the zodiac, without removing the old ones; and likewise we could add new signs to the zodiac without changing or removing the old zodiac, by placing the new signs on top of the old ones; in a way as co-signs, but which would have their own rulers.
There cannot be any new constellations to be discovered, but there may be new interpretations of the ones in existence. Generally there is a distinction between zodiac signs and constellations, and so the signs are fixed at 12 in number and of 30° each, while the constellations are 13 in number and vary in degrees. However, from 1970 and forward, new signs were suggested which would mirror a new kind of consciousness in humanity. We could not "make room" for them in the old zodiac, that would destroy the entire system; but we could place them on top of old signs giving them a greater depth.
Ophiuchus
The sign of the snake-bearer is in Roman mythology Asclepius, the god of healing, but some consider him also to be Apollon (father of Asclepius) and also a god of healing. The teacher of Asclepius was the centaur Chiron, and so it would be natural to assume the minor planet Chiron the ruler of Ophiuchus. Since Apollon is god of the sun, it would fall in its place to make the sign the element of fire. There has also been suggestions where to place the sign in the zodiac. My suggestion is this: The sun moves through the constellation between November 29 and December 17 (which has to be compared with the sidereal zodiac), and therefore coincides with the second and third decans of the sign of Scorpio, which in the sidereal zodiac is between November 16 and December 15. We could therefore interpret this that planets in 10°-30° of Scorpio are also in the sign of Ophiuchus, which we perhaps could also view as the Eagle-form of the Scorpio. Scorpio would then no longer be the darkest sign in the zodiac, but also the healer.
Cetus
A fourteenth sign has also been suggested; the sign of Cetus, which in mythology is a sea-monster, or a whale. Naturally this would attribute the sign with the element of water. Since Sedna is the sea-goddess of the Inuits, it would fall natural to make Sedna the ruler of Cetus. The sun moves through Cetus between May 12 to June 6, which in the sidereal zodiac coincides with the first two decans of the sign of Taurus. My suggestion is then: Planets in 0°-20° in Taurus would also be in the sign of Cetus, attributing a watery quality to Taurus.
Empirical Astrology
New ideas may be very theoretical, and so when changes are made to astrology, it would be necessary to study them empirically before concluding that they actually are real and true. Perhaps adjustments will have to be made, until everything come into focus and become clear. With so much change, so many discoveries, it demands a lot more from astrologers than it used to; however, as an art-form, it is always more fun with a few extra colours in the palette. The suggestions for new changes of course has to be thoroughly researched before accepted, and are at present time mere speculations and trivial ideas. Some may claim that the perfect symmetry of classical astrology becomes broken, but well, it was already broken with the inclusion of Uranus. Perhaps what is needed are sufficient additions to weld a new symmetry, in perfect balance again? Such a project is in progress...