I'll start with the plot, which I already mentioned would be an AU.
So far, the idea I have for the AU is this: during the Third War, instead of summoning Angra Mainyu as Avenger, the Einzberns instead decide to summon Amatsu-Mikaboshi, the Primordial Darkness and god of evil and the stars in Japanese mythology, into a unique container, Chaos. Hoping to use its power to devastate the opposition, and obtain the Grail, the Einzberns soon realize their mistake. AM (funny, same acronym) is too powerful to be bound, even with the Command Seals. AM only goal and drive is to bring the world and existence back into its primordial state of nothingness. Eventually, somehow, and involving the Grail, AM is defeated and sealed inside of it. However, it was too late, and by then a large disaster had occurred. This affects the world from then on, and forces changes to the Grail War system (read: justifies gameplay).
The villain's Servant, I was thinking, would be a Lancer (class may vary) called Ame-no-Kagaseo, who in reality is AM's human aspect/incarnation (like EXTRA!Caster to Amaterasu). At the end, the player would confront both the villain and his Servant, and after the two are defeated, the villain uses his Servant as a Catalyst to bring back AM, thus starting the game's final battle.
Also, I had the idea of making two different gender protags, a la EXTRA. May be silent, or not. It depends on how much it should affect the plot.
Now, for gameplay.
Well, first off, as I mentioned, there would be two protags a la EXTRA, one for each gender. The game would be divided into 15 chapters, or "Days", that chronicle the events that occur during the war. At first, the protag is by itself, but by the end of, say, Day 1, they already get a Servant. I'm still undecided whether we should give a choice, or to give one specific Servant.
Battles involve two teams. The player is restricted to a maximum of 8 characters in their team. However, the PC has to be in the party, there must be at least one Servant, and all Masters have to have at least one Servant assigned. So...a team can go from 1-1 Master and Servant, 1-7 Master and Servants, 4-4 Masters and Servants, and so on.
Characters would eventually be recruited into the player's party. I had the idea of Rin being the first character to join you, dunno why. When a Master joins, they bring with them their Servant. Oh, speaking of which...
To accommodate gameplay, there have been...changes to the Grail system. First off, there are more than 7 Masters. Also, Masterless Servants can make contracts with Masters, as always, but now, it's standard practice. The idea is to defeat the enemy Masters, take their Servants, and slowly build a strong team who can win the War. Now, all Servants have some kind of Independent Action, in that they can remain for a day even when their Master is killed. Servants can also be transferred between Masters, for the sake of gameplay. The actual skill Independent Action now has a different use in gameplay. I'll explain in a bit.
OK, back to battles. At the start, each Servant unit has to be assigned to a Master. Masters and Servants with good match-ups gain bonuses in their stats while connected. Those with bad match-ups, penalties. When a Servant is killed in battle, the Master can continue to fight. However, if the Master is killed, all Servants assigned to him/her have a few turns before they disappear (die). How long they last is defined by their IA skill (everyone has it). There could be an option for another Master in the team to connect with the Servants, but that's optional for now. If the Servant is masterless by the end of a battle, they have to be re-recruited. Again, this has to be done within the day, or else the Servant disappears and is lost forever. Obviously, dead Masters remain dead (a la Fire Emblem).
If the MC, or plot important characters (like say, Rin), die, it's game over.
Outside of battles, the player can explore Fuyuki, and visit different locations, such as the school, temple, etc. Some of these areas may also be used for battles, btw. Servants, even the ones in the party, don't have their identity revealed most of the time. There are exceptions (Ilya's Berserker), but for those who don't have it, you can look for info around the city. Discovering a Servant's true name is key in unlocking their strongest abilities/Noble Phantasms.
Servants will obviously come with their own abilities (and can learn some with experience/level ups), different classes come with different abilities, but for Master characters, they have to learn them through items, or some other method I still have to think of. Spells and skills have different effects, yada yada, I believe you know the drill. Some characters, though, will have unique abilities, like Rin's jewels or Shirou's projection. I also had an idea of maybe multi-classing Servants, like Lancer having "Lancer" and "Berserker" as options.
If possible, we could also add a relationship mechanic to the game, a la Fire Emblem, or maybe a la Persona. This could increase stats for Master characters, give them new skills, and in the case of Servants, increase their match-up with the characters. Yes, it would involve others, no only the MC.
Normally, through the story, you'll eventually get one of each type of Servant, along with their Masters. Other characters and Servants may be available to be recruited ("Kotomine joined the party!"), some temporarily, others permanently. Boss battles would involve Master and Servant teams, a Master with his Servant and some familiars, a Master with various Servants, etc. As per the rules, you can recruit the enemy Servants after the battle, unless the plot says no. Same with the Masters, sparing them can allow you to recruit them, too. Not all characters need to be fought.
Regular enemies could be some sort of Familiars, summoned creatures, or I-don't-know-the-hell-this-is type of enemies like in EXTRA. There could be events around the city, during certain days, for side-quests and recruiting and stuff.
I also had the idea of implementing some of the characterization from the series into gameplay. Like with Gilgamesh's Ea; it's the strongest attack in the game, bar none, but can only be used with 3-4 enemies in the entire game (Saber, Zero!Rider, etc), no matter what, because Gil only brings out Ea against those he considers worthy.
I still don't know how to implement the command spells, but they should go somewhere.
