For decades we only had one Lantern Corps, the Green Lanterns. The green power ring was powered by the will of its users and was said to be the most powerful weapon in the galaxy since it was powered by the most powerful emotion. The existence of the yellow power ring powered by fear was around since Green Lantern #7 in 1961, when Sinestro made his first appearance. Created by the Qwardians, Sinestro was the only one that used a yellow ring. Until 2007 when he created the Sinestro Corps.
Since then, we've had an explosion of colors and emotions. Little by little it was revealed that other Lantern and power rings existed. Each is fueled by a different emotion and is controlled by a separate Emotional Entity. The Guardians sought to keep them all concealed because they felt emotions were too dangerous. The power of each ring and the Corps for each that soon followed was too much for the Guardians to conceal.
Now that we have so many different Lantern Corps, is there room for any more? Are there any emotions or colors left that could be used?
Besides Green Lantern and Sinestro, we also saw the Star Sapphire Corps. Using the color pink (violet) and the power of love, the Zamarons left Oa billions of years ago when the Guardians decided that life should be without emotion. They created their own Corps focused on love.
The Red Lanterns were born when Atrocitus used the blood of four remaining survivors of Sector 666 (which was wiped out by the Guardians' first police force, the Manhunters) along with some magic rituals. This Corps is all about rage and hate.
There really isn't an Orange Lantern Corps because the sole member, Larfleeze won't allow it. Larfleeze came into possession of the Orange power battery billions of years ago. As a former thief, Larfleeze was given the opportunity by the Guardians to keep the battery as long as he stayed within the Vega system. They figured that would be the safest way to keep the battery from spreading across the universe and by only allowing one to possess it, Larfleeze would ensure it didn't leave his grasp.
The Blue Lantern Corps was formed by banished former Guardians Ganthet and Sayd. The emotion that powers their ring is Hope. When the Sinestro Corps War was going on and the looming Blackest Night prophecy, Ganthet and Sayd created this Corps anchored on that tiny bit of hope that all will be well.
The Indigo Tribe uses a combination of their power rings along with a power stave. Their ring is fueled by compassion. The Tribe isn't very forthcoming with details to their history. The members have abandoned everything and dedicated themselves to compassion. The only thing we really know is the Tribe's leader, Indigo-1 was once incarcerated by the Green Lantern, Abin Sur.
The Black Lanterns get their energy from a darkness that existed before the creation of the universe. The members are all deceased and have only one goal, to eliminate all life from the Universe.
With nine different Corps already, it looks like all the colors and emotions are represented. There's just no way this is all of them. If these other rings existed all this time, or if the potential for them existed, there has to be others as well.
What emotions are left? When looking over what we have already, I couldn't help but think of the seven deadly sins: gluttony, greed, anger, sloth, envy, pride and lust. Some are already represented by existing rings. Gluttony, greed, and envy are similar to avarice, which is the Orange ring. Anger is the Red Lanterns. Lust could be related to Love. That leaves Pride. Pride could be associated with Will. Having pride in yourself, believing in what you can do, that could be a trait similar to what the Green Lanterns have. They've seen themselves as the police of the universe, regardless what any individual planet or system might want.
Are there any emotions left that could be a suitable focus for another Lantern Corps? There's sadness, disappointment or despair. Maybe guilt or shame could work. The problem with these emotions is if someone is feeling them, they might not have the motivation or will to channel their emotions into powering a ring. And what color is left for them? Brown? I like the color brown and have been known to wear the occasional brown shirt or pants but brown is also the color of poop. Would someone really want to be a Brown Lantern? Then again, if that was a sad thought, it could be used to power their ring.
Other emotions that could be used to fuel a Brown power ring could be despair, guilt or shame. If a person had a lot of shame, they could try to envision themselves in an embarrassing situation. But would they be a force of good or evil? If they decided to use their power in order to get revenge against anyone that made them feel sadness, wouldn't that make them more sad? They would have more shame or guilt for abusing their power. Or if getting that revenge made them happy, would they end up losing the focus needed to power their ring?
If we wanted to go the Pokémon route, we could also have a Silver Lantern or Gold Lantern. Since those are flashier colors, we'd need a flashier emotion to go with them. Could happiness or joy be enough to power a ring? Those emotions would need a bright and cheerful color. Would gold or silver suffice? There's also the possibility of having a a variation or different shade of a color already being used. The problem is Neon Yellow Lantern Corps or Atomic Tangerine Lantern Corps just doesn't sound right.
Do we already have enough Lantern Corps? Is there room (or colors and emotions) for more to be created? The universe is a large place so these other Lanterns might already exist. It's not like the Guardians are always truthful in revealing everything they know.
From a marketing perspective, DC would love to have more Corps so they can sell more colored t-shirts. From the comic's perspective, since we now have nine different emotional entities, it would be silly to think there weren't any others out there somewhere.