While you cannot find any anime that is exactly similar to Code Geass to fill that awesome void you get after watching it, there are some really interesting anime that could also capture your heart. Below are some anime recommendations for the mecha genre with awesome plots and characters.
Death Note is such a popular anime and it is still sought after today. The story revolves around a boy named Light Yagami. One day, he found a powerful note book that could kill anyone as long as you knew his name and face by writing on it. It was dropped by a Shinigami(Death God in Japanese) named Ryuk.Upon knowing what he could do with the note book, he decides to create a world based on his own version of justice. He starts using the note book his way, killing off criminals that he deemed was not appropriate for the world. Soon enough, unwanted criminals start dying off mysteriously with a "heart attack". Soon enough, a detective named "L" is assigned to find out the cause of the mysterious deaths.
Full Metal Panic!
This anime has mecha, romance and also military genre. The main protagonist is Sagara Sousuke, a young and talented military geek of an organization called Mithril. He gets sent to protect a whispered human called Kaname Chidori. Together with his comrades Melissa Mao and Kurz Weber, Sousuke goes undercover in Jindai High School. What is cool about this anime is that they incorporated comedic scenes in between serious mecha and military fighting scenes. Being a military geek, Sousuke is extremely inept in comprehending what being a normal high school student is like. He messes up the simplest of routines and situations, causing havoc in the school.Chidori is unsettled by the sudden appearance of Sousuke, but as the anime goes on, Chidori realizes that Sousuke is sent to protect her from someone and she appreciates it, but rarely shows it. The reason why Chidori is targeted because she has a special ability that could activate a powerful system...
Gundam 00 is the 11th version of the long Gundam franchise. The story starts in 2307 AD, where fossil fuels are depleted and humans are depending on a new source of power for survival. It comes in the form of Solar Orbiters around the Earth. This form of energy is inexhaustible, making it an ideal fuel for many countries. This has caused war to erupt in different countries.Countries that used to depend on fossil fuels are now in poverty because of the new form of solar fuel found. Due to the constant warfare and chaos, a private organization called the "Celestial Being" is formed. Their goal is to eradicate war with the use of 4 powerful mobile suits known as Gundams. Setsuna F. Seiei, the main character is assigned to pilot one of the Gundams. Together with 3 other "Gundam Meisters", they fight against the United Nations Army to achieve world peace through the power of their own Gundams.
This anime has a plot rather similar to Code Geass and it has some cool fighting scenes. Before the start of the story, a biological hazard called "Apocalypse Virus" hits Japan. Being unable to contain the situation, Japan asked for help from the United Nations. An organization called GHQ was sent to aid Japan and succeeded in containing the virus. This came at the price of Japan's independence. 10 years later, a rising organization, Funeral Parlor decided to rebel against the GHQ to retake what was originally their own country, the old Japan.The main protagonist of the story is Shu Ouma. One day, he helped a girl named Inori Yuzuriha of a singing group called EGOIST. Lo and behold, he does not know that he had been helping a member of the Funeral Parlor from the GHQ. With the help of a powerful weapon that was created by the Apocalypse Virus, granting him the "Power of the Kings", and he was able to defeat the GHQ mechs that were trying to capture Inori...
This anime has unique mecha designs. The story revolves around a boy named Simon. He lives underground together with other humans as Earth is ruled by the Spiral King. Since the underground villages are always threatened by earthquakes, diggers are required to dig the ground for a better place to live. Being a small digger, Simon is often despised by his villagers. Thankfully, he found a friend in Kamina, a delinquent with very eccentric behavior. Together, they decided that they want to dig up to Earth and see what it is like up there.One day, Simon dug up a mysterious robot known as gunmen. Coincidentally, a gunmen piloted by a beastman crashed from above. Together with Kamina and a newcomer and beautiful girl named Yoko, they managed to defeat the gunmen with the robot they named "Gurren Lagann". They broke through the surface and their journey begins...
This Gundam series takes place in Cosmic Era, a different universe from the previous Gundam franchises. There are two kinds of races in the anime. The first is the Naturals, people that live on Earth. The second is Coordinators, genetically enhanced humans that live in space colonies. The story starts in Heliopolis, a neutral space colony where a secret operation is conducted to build powerful mobile suits known as Gundams to aid the Naturals to declare war on the Coordinators.Kira Yamato, a Coordinator student living in Heliopolis, is thrown into the midst of a terrible war that is about to happen. The colony came under the attack of ZAFT forces, who are there to steal the mobile suits that the Naturals made. Forced to evacuate, he eventually ends up piloting one of the Gundams known as GAT-X105. Before that, he encountered his close childhood friend named Athrun that he was separated from when they were young. He was assigned to steal one of the Gundams...
These anime like Code Geass definitely gives CG some competition and perhaps you might also fall in love with some of these anime. I highly recommend Full Metal Panic! if you enjoy plenty of comedy and action-packed mecha fighting scenes. The others are also not too shabby as well.