Here are my favorite picks for anime like M3: The Dark Metal. Keep in my the list is in no particular order. Enjoy the recommendations!
In the year 2015, the Angels, huge, tremendously powerful, alien war machines, appear in Tokyo for the second time. The only hope for Mankind’s survival lies in the Evangelion, a humanoid fighting machine developed by NERV, a special United Nations agency. Capable of withstanding anything the Angels can dish out, the Evangelion’s one drawback lies in the limited number of people able to pilot them. Only a handful of teenagers, all born fourteen years ago, nine months after the Angels first appeared, are able to interface with the Evangelion. One such teenager is Shinji Ikari, whose father heads the NERV team that developed and maintains the Evangelion. Thrust into a maelstrom of battle and events that he does not understand, Shinji is forced to plumb the depths of his own inner resources for the courage and strength to not only fight, but to survive, or risk losing everything. (Source: AniDB)
During a summer camp, 15 children, 8 males and 7 females, find a grotto by the sea. Deep within they discover working computers and some electronic equipment, and later the owner, a man called Kokopelli. Kokopelli claimed to be a programmer working on a brand new game, in which a large robot has to defend the Earth against alien invasions. He persuades the children to test the game and sign a contract. All but one of them sign, barely a moment later they mysteriously awaken on the shore believing what happened was just a dream.(Source: ANN)
A millennium from now, in Japan, exists a utopia. The protagonist, Saki Watanabe, lives in an idyllic village barred from the outside world. Her world is ruled by the people who possess the “gods’ power” of psychokinesis. After finally obtaining her own powers, Saki enters the Zenjin Academy to train along with five other children: Satoru Asahina, Maria Akizuki, Mamoru Itou, Shun Aonuma, and Reiko Amano.
Not all is as it seems, however. In this utopian village, strange rumors about a monstrous cat that abducts children circulate, and students are said to disappear from the academy. The world and its history are much darker than they appear and humanity is on the verge of collapsing.(Source: TV Tropes, edited)
In the year 2010 AD, Hyper Hurricanes born concurrently all over the world caused severe damage. Five billion people lost their lives, decreasing the world’s population to 3 billion. Existing economies and military were wiped out. Countries were frozen under solid ice, plains sank beneath seas, the world was changed dramatically.
Human civilization enters a chaotic era. Poverty flourished outside of these economic zones and slums were widespread. The ruling class called themselves Logos and maintained their position by force of arms. They call the poor Revenus, who are exiled to live outside the special economic areas.
Time has passed since then. Katsuragi Jo and Tsurasawa Jin, escape from Phantom, a force organized to watch Revenus and to suppress renegade elements of the Logos, taking with them a mysterious girl, Nobuto Sana. There are many who are interested in her, all with their own reasons.
The Gastrea virus spreads like wildfire, causing each infected host to rapidly mutate, gaining new powers and abilities with every stage of development, even while they continue to attack, kill and infect multiple new hosts.
Unable to fight a pandemic enemy that turns defenders into adversaries, the shocked remnants of the human race are forced to retreat into cities surrounded by giant monoliths made of Varanium, one of the few materials that can stop the Gastrea. Now mankind’s last hope lies in the Cursed Children: young girls infected with the virus who have managed to retain a hold on their humanity. Paired with a partner, they alone have the strength to take on a Gastrea in one-on-one combat.
But the very thing that gives them power is a ticking time bomb, so even as they protect humanity, they are also feared and shunned.(Source: Sentai Filmworks)
It’s been a thousand years since the Gauna, a strange alien race with no known method of communication, destroyed the solar system. A portion of humanity managed to escape using enormous “seed ships” like the Sidonia, which have allowed them to maintain the population while drifting through space. Nagate Tanikaze is a young man who has been raised deep in the bowels of the ship. When he goes into training to pilot the huge robotic weapons known as Gardes, Nagate is entrusted with piloting the legendary unit known as Tsugumori. Nagate and his fellow pilots put their lives on the line against the Gauna, in the ultimate battle for the survival of humanity!(Source: Polygon Pictures)
The year is AD 2225. Kouji Aiba and Aoi Housen are serving as astronauts in-training in Liebe Delta which is located on the edge of the Geduld Sea. When saboteurs with unknown intents suddenly strike during a routine dive procedure, the space station plummets into the Geduld, a plasma field that links all the planets like a nervous system and crushes any ship that strays too far into it. With all the adults onboard killed, the young astronauts will have to survive this long journey home in midst of the growing tension amongst each other. Meanwhile the organizers of the sabotage look on and prepare to attack once more.
References: MyAnimeList, Wiki, ANN