Medieval Linear RPG

I came up with this idea not too long ago, and I'm really looking into doing this.

The (very simple) gist:
The year is 1765. Hell has broken loose, and demons are rampaging across the land. You, and the Organized Forces of Earth must beat them back to hell, closing the holes they used to escape to earth in the process.

What I will have to do:
The 3D Modeling for the demons, weapons, other npcs, the player character, etc...
As I will not use prefabricated controllers, ALL the coding

What do you think? Sound like a good idea?

 

I think it's too broad for me to be able to tell you much of anything.

 

More in depth:
The main character (no name yet) is a knight of the British forces. You are one of the crusaders. You start out in a bloody battle with other humans (going to go for a historical battle here), but in the middle of it demons break out of the earth and wipe out nearly everyone. You escape back to Britain to warn the Queen. But when you return, London is already destroyed by the demons. You rush to the castle to check for the Queen, fighting the demons along the way. When you get there, the queen is already dead, along with everyone in the castle. For the first 1/3 of the game, you and the rest of the survivors of the Crusaders are fighting both the demons and the human enemy, until the world realizes that they can not win against the demons as fractured as they are. They join forces and fight back, going to major cities of the world, closing holes that the demons have crawled through. At each hole, a major demon (from real-life mythology, and from Christianity) is guarding it, obviously with their own special attacks, etc... and after you kill them, you must destroy the hole as well. At the final hole, instead of closing it, you travel into hell, and fight Satan himself, killing him in the process. You are hailed as the hero of the world.

Other things in it will be a sort of base camp between each linear quest, where you can follow a holy or unholy path. Holy path entails going to church etc..., and unholy will be the opposite. If you follow holy path, you can call in the forces of Heaven to help you in battles, and other various powers, like a fiery sword, etc.. while the unholy path makes you physically bulkier and cause more damage to the enemy demons. There will be various lesser demons, some from mythology/Christianity, some designed by me or my friends.