IMPORTANT: This has to be a direct recycling of a premise, or follow up to a work. Simply having some similarities to another show is not this trope.
Say there is an old fairy tale about a good spirit that has to stop a horde of The Fair Folk from invading a village. Then someone makes an adaptation where the fair folk are aliens that pilot giant robots, and the good spirit turns out to be the only one who can pilot the Super Prototype and save Earth.
Note that almost all other mecha related tropes can apply, as long as the previous/source work did not have mecha.
- Str.A.In.: Strategic Armored Infantry, based on A Little Princess.
- Gankutsuou, based on The Count of Monte Cristo. Duels are fought not just with pistols or swords, but with six-foot long swords wielded by Humongous Mecha.
- Samurai 7, based on Seven Samurai.
- This is pretty much the entire point of Idolmaster: Xenoglossia, where The Idolmaster was basically a talent show sim.
- Kannazuki no Miko was actually based on an older Kaishaku work known as Cross Triangle, but now with giant mecha named after Japanese gods.
- The Dating Sim Graduation had an anime OVA follow-up, Sailor Victory, which was about mecha with ninja abilities.
- The Lyrical Nanoha franchise is an oddly gradual version of this. While it started a Magical Girl Warrior spinoff (to Triangle Heart 3: Sweet Songs Forever), it added more and more mecha elements and Shout Outs over time, culminating in Magical Record Lyrical Nanoha Force where the characters are military officers using Powered Armor.
- Knights of Sidonia is essentially Attack on Titan but with giant mecha and in space. Note that the original already has mecha (sort of...)
- The Big O has been called Batman: The Animated Series with Humongous Mecha. To clarify: it has been called that in official [adult swim] promotion materials.
- Japanese Spider-Man, when Spider-Man received various tropes that would eventually be absorbed into Super Sentai (Specifically Battle Fever J onwards, as stated later). Spider-Man gets his powers and costume from an ancient alien race, and gets a car and a giant robot to go along with them.
- Battle Fever J to the first two entries in the Super Sentai franchise, inspired by the Spider-Man example above. Afterwards it became a standard part of the formula.
- BattleTech, the Trope Codifier for the Western Mecha genre, is essentially medieval Europe with Humongous Mecha-piloting knights in space. Despite two of the Great Houses being mostly Asian. The Star League is the Roman Empire, the Successor States are the assorted kingdoms of Western Europe (the Federated Commonwealth may qualify as the HRE), CommStar has influence similar to the Roman Catholic Church, and the Clans are the Hordes from the East.
- The CAMELOT Trigger setting for FATE Core is "King Arthur, except in mech suits fighting evil robots in space."
- DragonMech is Dungeons & Dragons only with Steampunk mecha.
- Invoked and justified in Chris Perrins' Mecha. The core rulebook comes with three stock settings and the explanation that GMs should create theirs by watching things other than mecha anime, to make it more interesting. Special Research School takes a significant amount from American Graffiti while Godblind comes from Dark City.
- The wargame Gear Krieg is World War II with Dieselpunk mecha (called "walkers").
- An Australian exam didn't pay attention to the illustrations it was using, leading to an alternate October Revolution where the Russian proletariat had heavy mech support to storm the Winter Palace.