I know, I promised to finish this yesterday, but I got struck by an awful lack of time and a lot of confusion about stuff. Managed to finish the whole concept just now. So, here's the idea:
Basically it's an open-world game, a bit like Skyrim. You start as a Duelist (male or female) with one Duel Spirit, a Spellcaster you pick from a list (you can choose from ten to twenty Spellcasters) - you can get more Spirits, ten at most, in your party as you progress through the game. There are two worlds - the Human world and the world of the Spirits - and there are corresponding main areas in the game, which, by the way, also refer to elements from the YuGiOh series up to and including 5Ds.
The main areas in the Human world are the Forest (exactly what it says), the Hills (a range of hills and a few small mountains), the Plains/Wasteland (one area consisting of 2 parts with the Plains being grassy and the Wasteland being barren, almost like a desert - see what I did there?), the Ruins (there are several areas referred to as Ruins with one being located in the Wasteland and having an Egyptian theme, another being located underneath the City and representing a post-apocalypse version of the City, the third being located in the Forest and the Hills and having an Aztec/Mayan theme, and the last one being located on an island you can only reach through the Spirit world - there is no way to reach it through the Human world unless an important character takes you there - which has a mixed Ancient Culture theme with mainly Greek culture), the Beach (the Docks are located here as well), the City (there's three areas in this; the almost completely abandoned Slum where the Graveyard is located as well, the futuristic Metropolis with courses for Turbo Duels, and the 'normal' City), the Academy Island (which can only be reached by taking a boat at the Docks), and the Tournament Island (this can only be reached by taking a boat at the Docks, the boat will only be at the Docks on the first day of every month but you can leave the Island at any time you want, you just have to wait until the next month to go there again).
The main areas in the world of the Spirits correspond with the Field Spells in the game and roughly correspond with the areas in the Human world as well. It's like the Light and Dark World in 'Legend of Zelda: A Link to the Past'. Certain monsters can only be found in certain areas which correspond with the location of their typical Field Spell. For example, the Zombie monsters and most Fiends can only be found in the area corresponding with the Slum and mainly the Graveyard in the Human world, while the Naturia monsters are only found in the area corresponding with the Forest. Your first Duel Spirit, the Spellcaster you picked at the start, is able to open a portal between the two worlds at any time in the game, unless you're inside a building - in the Human world, the Ruins do not count as buildings, unless you are in the Palace Ruin (Wasteland), the Temple Ruin (Forest/Hills), a Damaged Skyscraper (City) or any building that still has a roof (special Island). In the Spirit world, there is no place where you could get confused with inside/outside so that is easy.
We're making it even more confusing: If you're inside the Ruins (Wasteland or Forest/Hills) and you have completed certain plot elements and have a Spirit related to that location with you (any Spirit from the Millennium World Arc for the Ruins in the Wasteland and any Signer Dragon for the Ruins in the Forest/Hills) as well as a Time Magician, you can travel in time to the past by letting the Time Magician create a portal. Yup, you can end up in Ancient Egypt or even further in the past. And it gets even better! If you're on the island you can only reach by passing through the Spirit world and you have a Time Magician with you and own a certain item you can only obtain in certain plot parts, you can end up in Atlantis by letting the Time Magician create a portal. Last but not least, the Ruins below the City can be accessed at any given moment without any special trigger condition, but are actually located in a post-apocalyptic future of the actual City. The only way to get back to the present from any Ruin (except for the City one, you can leave that one whenever you want) is to go back to the portal the Time Magician opened. If a portal has been opened, it will stay open and you can pass through whenever you want, as long as you have the necessary trigger monster or item with you.
Okay, enough about the locations. Let's get to the more important things - character designing and plot. Character designing goes first. It kinda works like it does in the Elder Scrolls games. You pick a gender and you can customize everything else, from build to skin color to clothing style to age. Yes, you can change the character's age within a certain range which more or less matches the age range of the characters in the series. The youngest a character can be is 12 (5Ds; Lua and Luka are 12 when the WRGP arc starts, and Lucciano is the same age in that arc. just a little trivia), and the oldest a character can be is 25 (DM: did you know? Pegasus is actually 24. DM & 5Ds: I'm guessing some characters whose age is unknown are between 20 and 25 years old), to fit the age range of the important characters in the series. You can change a character's clothing and hairstyle during the game if you want, like in Pokemon X and Y. Depending on the gender you picked and the plots you play, characters may or may not show a slight crush on you, and you can choose to try and win someone's love as well (warning; some characters are MUCH more difficult than others) if you really want your fangirl/fanboy crush to like you.
And now, at last, PLOT! The plot is actually pretty odd... Just like in Skyrim, there is one main plotline, with a lot of sideplots. The main plot is very simple as well - Something is threathening both humans and Spirits, and you have to help find a way to stop this 'something'. Then, there's a bunch of plots that each have something to do with the 'something' (that sounded SO odd XD). Anyway, you have a lot of freedom to go wherever you want - you can decide to join Yliaster or Doma, you can try to become a Card Professor at I2, you can participate in 'normal' Dueling tournaments, you can get (and style!) your own Duel Runner for Turbo Duels, or you can just befriend as many Spirits as possible and explore around the worlds. There are hundreds of options, really, and only a few options actually cancel each other out. Even if you decide to join Doma or Yliaster, you can decide to leave them again - just be prepared to be hunted down by them. They won't let you go so easily.
Oh, right, forgot to address one important point - Which characters are in it? Well... Basically everyone from all series up to and including 5Ds. This includes the anime-only arcs and the YuGiOh R manga. ZeXal and Arc-V expansion packs can be made separately and add some more characters and plotlines, but will have no major impact on the game - just a few more sideplots. As for the Duel Spirits, I'm not sure how many would be included. Probably every monster we see somewhere in any of the series mentioned earlier in this paragraph, and maybe a little more of certain types to have every type accurately represented in the game. It's difficult to not play favourites with something like this...
Well, I think that's it for now. If you have anything you want clarified or think should be in the game, Comment here and I'll see if I can include it!