A lot of post-apocalyptic thrillers with zombies, WWII fanatics, remakes of old installments, and other little gems (like the Eric Harris idea)...I like this thread. As such, I'd like to add a few ideas of my own:
I had a rough outline (from character bios to specials, combos, and super moves) of a 2D fighting game, in which characters had different styles depending on which type of music. For example, there was a burly, fire-haired dude with a sleeveless leather jacket, jeans, and boots who utilized heavy metal (blunt, brutal combos and a few energy attacks), a young, shirtless male dancer who focused on agility and combos in hip-hop, a female with long-flowing hair and a fur-lined trench coat who could construct lethal energy compositions and wiry defense in an old world orchestral-type style, and so on. At the time (many moons ago, in my early teens), it had a typical tournament feel (basically beating everyone else to get to the final boss; someone very powerful and cheap), but I tried to develop it beyond that to much failure and lack of motivation. This was before the craze of Rock Band, Guitar Hero, and Dance Dance Revolution.
Another idea I had is a crossover of Guilty Gear and Psychic Force, utilizing the 3D engine of Guilty Gear 2: Overture, minus all the Dynasty Warriors-esque bullshit, plus the seamless controls of its 2D predecessors. For those of you who are not familiar with Psychic Force, it is an obscure fighting game which pitted a group (a rather small, broad group of stock but potentially awesome characters) in a cube to kick the crap out of one another with ranged projectile attacks or close-range combos/throws. Very unique for the time, in my eyes, and had a deep strategic aura to it. Too bad Taito couldn't develop it further (to be fair, I read somewhere that Taito didn't have enough experience making a fighting game, but they learned fast in the second Psychic Force game, only in Japan; a Sega Dreamcast version called PF: 2012 (how ironic, with all the hype of the apocalypse coming on this year) to American markets).
Anyway, I would revamp the excellent three-branch Story Mode from GGX2 (not Accent Core; the original version, before it 'Capcomsized' everything with endless versions of the same game with minor new features) and expand on that with the Guilty Gear and Psychic Force cast. Since there is very little word on a new Guilty Gear game beside Overture, I would go forth where X2 left off.
The main plot would have to do with Richard Wong, a powerful individual who controls time in Psychic Force, who (in my creative hands) is able to cross over from 2012 to the 2200s (the time period where GG is taking place) and capitalize on the chaos created by I-No, the rock-n-roll witch in red who acts as his confidant (so the witch is a double or triple-agent of sorts...there's so much to the GG plot, so I'll try to make it as simple as possible). Also, there is a link between the Gears and the Psychics (Gears are physically, biologically advanced, while Psychics have intense, often erratic mental abilities).
Behind the scenes, the Psychics are of high importance to a select few (including I-No); Wong developed a highly complex machine, using technology from the GG universe, to squeeze the time continuum into frenzied proportions and affecting those who are Psychics, and hurling them into the GG universe.
This is just rough stuff, but as for the in-game combat, players can customize their fighting style from the GG and PF control-scheme camps:
For Guilty Gear, players can tap into characters (from either GG or PF) with features like Gatling Combos, Air Dashes, Faultless Defense, Roman (and False) Cancels, Psych Burst, Tension moves, and Instant Kills (to name a few). Players can execute normal attacks (w/Punch, Kick, Slash, H-Slash, and Dust) and specials all they want, and it builds up the Tension gauge. FD, RC/FRCs will drain the Tension gauge by fixed intervals, the Psych Burst (if connected) can fill Tensions, and players can execute a flurry of physical attacks and be able to cancel into Overdrives. Instant Kills use up the entire Tension gauge once, so if you fail, NO Tension for you. More of an offensive, combo-tastic slaughterfest.
For Psychic Force, players will utilize a more strategic approach with features like Emergency Barrier, Dashing (on ground or midair), Slide Dashing, Psycho Charge, Hyper Charge, Avoidance Barrier, and other features. Combos are limited to Light and Heavy attacks (the other buttons will guard, shoot a regular projectile, and a Special Range attack). The Psy-Gauge is set at 100%, and each Special you execute (say you wanted to toss out a Gun Flame, or Stun Edge, a Mega Fist, or any other GG-related special; it would cost you 20 to 30% of your gauge). The more damage you take, the more energy is put into the Psy-Gauge (for a maximum of 200%). The Emergency Barrier, Psycho Charge, Hyper Charge, and Avoidance Barriers affect portions of the Psy-Gauge (EB drains it slowly, PC charges the gauge, HC decreases vitality to strengthen overall power, and AB drain 50%). More evasive, planning, and trickery.
What about characters like Axl, May, or Slayer? How would the projectile properties work for them? Simple. They'd have extra techniques to compensate for those without projectiles or different other areas.
One more thing (for now): the player can mix up their own fighting style based off different aspects of the GG and PF styles. For example, say you enjoy executing Gatling Combos and Roman Cancels, but want to add a little bit of tactics for aspects like Psycho Charge or regular Dashing...it's possible to do that. Or, you could go all the way for one type or the another.
I am more enthusiastic about the PF/GG crossover, as I hope you all can tell. I will try and contribute more in the future if the interest is moderate enough.