Consider Using My 'LBX Girls' (AU)/'Girls' Frontline' (AU)/'Log Horizon' (AU) Idea
Consider using this idea I posted here on the SpaceBattles forums:
LBX Girls (AU) × (maybe?) Girls' Frontline (AU) × Log Horizon (AU)
(Descriptions are, at least in part, loosely adapted from a few sources, namely including MyAnimeList entry descriptions, TV Tropes pages, and Fandom wiki pages. Before you ask: yes, I am aware that a proper premise/plot setup blurb for each setting would be better; this is just the best I could come up with at the moment.)
Riko Morisawa, a young girl from Tokorozawa, goes on a trip to Tokyo with her friend Mana. While they're out and about roaming around and browsing Ikebukuro's shops, Riko spots a billboard advertising an event for the 'Little Battlers eXperience,' or 'LBX,' line of mecha miniature figurine toys, a popular item across Japan that Mana's also a fan of, that day at a local mall. Riko's one of the few people who hasn't really gotten caught up in the craze and doesn't really care for LBX figures, but Mana suggests that, even if Riko doesn't want one, maybe she could get one for somebody else, like perhaps her dad. She doesn't initially see any that interest her but then sights discount variety-pack gift bags on sale and figures it wouldn't hurt to buy one. Tepidly/tentatively curious what all the fuss over LBX miniatures is about, Riko takes the one labeled 'LBX Assassin' out of its box to look at it for a bit. The next thing Riko knows, an interdimensional 'temporal rift' opens up underneath her and sends her flying into the sky of an alternate Japan.
While she's falling at terminal velocity, an exosuit based on the LBX Assasin materializes around her, saving her from falling to her death. Once on the ground, however, she encounters an unfamiliar post-apocalyptic, partly feudal scavenger world overrun by monsters and rogue, AI-powered autonomous, humanoid robots where the locals are as likely to survive on the surface wielding medieval-style weapons like swords, spears, and bows as they are to have managed to hold out in bunkers, supply depots, military outposts, and naturally-defensible positions and get any old technology, including LBX suits, or even just parts of them, and guns, up and running and staying that way. Fleeing from the hostile fauna indigenous to the time-scarred concrete jungle where she arrived, Riko finds herself rescued from her pursuers by a mobile unit of four other LBX-equipped girls who also got stranded here away from their own versions of Earth*: Kyoka Shinohara, Yui Takito, Miharu Kuruki, and Suzuno Otamaki. Upon learning of Riko's predicament, the combat group decides to recruit her into their ranks; anybody who can pilot an LBX suit is a valuable asset both in an urban wilderness teeming with living threats like the one where Riko first finds herself and beyond, no matter how unskilled they may be.(* It might work better to make these alternate Earths at least slightly less drastically different from each other; for example, in LBX Girls canon, each Earth's Japan has an entirely different capital; this could be nerfed to have the Prime Minister or a government official who's important either nationally or just famous locally for getting that high up the political ladder.
Alternatively, all the girls could come from the same Earth in this continuity.)
Long ago, humanity waged a harrowing, brutal protracted war with chemical, biological, and nano-scale technology; artificial battle beasts; and AI-powered autonomous, humanoid robots called 'Tactical Dolls,' or 'T-Dolls' or just 'Dolls' for short. Widespread conflict devastated the world's population and rendered large tracts of land uninhabitable. One of these T-Dolls went rogue during hostilities or not terribly long after they ended, taking control of all autonomous forces in industry-leading T-Doll manufacturer Sangvis Ferri's client's or clients' chain of command and lashing out in belligerent acts against opposing factions which continue to this day.
Fast-forward to the present: civilian and armed holdouts descended from survivors of that long-ended war have finally emerged from scattered safehouse bunkers and strongholds to begin the arduous task of restoring civilization…only to find the feudal surface already scattered with new human settlements amidst the scars and relics of past devastation. Mercenary group Griffin & Kryuger (often shortened to just 'Griffin') is one of many private military companies contracting their services out to protect suitable clients from any threat. They find themselves hired to patrol surrounding areas for what pieces of said hostile army of mechanized simulacra still linger on in this new world.
Griffin company's Persica has sent its AR ('Anti-Rain') team out hunting for crucial clues on how to do better against Sangvis forces. Its members have just found highly valuable, confidential intelligence in their search that they need to return to headquarters in an abandoned warehouse when they're taken by surprise in short order and ambushed just as they try to retreat from that position. Hoping to keep their critical findings secure, the group's members temporarily scatter and each go into hiding on the run. Meanwhile, Griffin command hires the lazy but gifted Gentiane as a rookie commander and gives her an important first mission: track all of AR team's Dolls down and recover the classified data they risked themselves to guard before Sangvis Ferri can.
On the release of its 'The Novasphere Pioneers' expansion, thirty thousand bewildered Japanese players of the established MMORPG Elder Tale suddenly find that they've been whisked away from their everyday lives and inside its world of magic and fantasy grown and layered over the remains of earlier civilization in the blink of an eye, leaving them unable to log out. Among these is Shiroe, a socially awkward college student, a long-time veteran of the game whose confusion and shock at his strange new shared situation lasts only a moment; seeking information and the opportunity he suspects awaits him in his new surroundings, he immediately sets out to explore the limits of his new reality.
Joining Shiroe on this unexpected journey are, among others, his friend Naotsugu, who was unfortunate enough to have been logged in when everyone got transported to this alternate Earth, and Akatsuki, a petite but fierce (and fiercely loyal) assassin who pledges her undying fealty to our beloved 'devil in glasses,' taking him as her master after he does her a major favor. Shiroe must learn to live in this new world, leading others and negotiating with the locals — some former NPCs, others new faces — in order to bring stability to his starting area, the previously-virtual city of Akihabara (with obvious namesake from his old world.) Follow a master strategist as he navigates a landscape of fantasy, adventure, and politics in an attempt to make the best of a difficult ordeal.