I would make a first/third person action/adventure/fantasy/RPG/strategy game! It will be a first/third person interaction I call Micro-view on one level, with gameplay like GTA V and Skyrim put together. Then there will be a global interaction I call Macro-view, that your character can achieve at a certain level depending on one's character, influence, skills, finances, and reputation or through conquering and pillaging - this gameplay will allow players to create things on mass such as build and maintain populace centers [villages, towns, cities, etc.], create armies, trade, and pretty much do what most strategy games are about.
First/third person gameplay has no limits to where one can go, while macro gameplay will be restricted in movement and interaction either bonds [oral or written; requires high influence, charisma, or good social skills], barter, or some form of financial instrument e.g. money. Macro-view time and processes can be made faster by collecting crystals that bend time-space, which are then used at temples that the micro-view missions will involve and solve, but this has a negative effect on one's character due to random growth algorithms. Micro-view interactions is how one influences the rise and fall of the Macro-view gameplay, because many missions will require working with other online players or NPCs to open Macro-view achievements.
The whole game will be on a few continents with hundreds of ocean islands. Depending on one's character creation, one will either find themselves starting on the continent or a island. All places are conquerable, so long as the reigning NPC or player is deposed - however, this is no guaranteed one will be accepted as the new leader if one's reputation is too low. There will be three ages that can be played: a prehistoric [Atlantis, giants, Dinosaurs, mythical creatures, etc.], a historic [Mythological deities, demons, vampires, trolls, orcs, etc.], and a present time period in which a mixture of the two previous ages of creatures seep through time-portals and are overrunning the human world!
I can't go any further, because I thought most of this on a fly, but I have had a similar game idea for a few years now. And yes, people can use my ideas, although I would like some recognition for inspiration or whatever.