I have a brilliant game idea of a Grand Theft Auto style game set in the prehistoric era, or a combination of prehistoric, ancient and medieval eras. more appropriately named. 'Grand Theft Horse'. You may think that this may be dull but, I have Ideas of the 'vehicles' that will be used. Not just horses, but also horse drawn carriages, open cargo carriages or stage coaches(passenger carriages), chariots, oxen, donkeys, camels, elephants, ostriches, (yes, you ride an ostrich). and sea vehicles like canoes, outriggers, boats, triremes, sailboats and pirate ships. You can jack any vessel or vehicle, travel between cities or villages of different cultures, and have weapons and armour of these cultures, swords, spears, bow and arrow. Cultures range from cavemen to medieval . You can commit crimes and get wanted then swordsmen and bowmen run after you. Cities range from villages with grass roof huts, to teepees, to medieval walled towns.  There will be missions to do similar to any GTA title. Brilliant Idea no one has thought of yet, isn't it?

Ideas on their own are worthless. Everyone has ideas, but not everyone can implement them. If you were to have a working prototype though, then yes, it might be cool, or it might not be. It all depends on the execution. Average game idea executed correctly will be better than a great game idea executed badly. 

Someone might take this idea and develop a fun game with it, and take all the benefits.

look into assassins creed origins specifically. I thought about that game instantly when you told us your idea (open-world in ancient times, you travel by horse, boat, carriage etc and do missions all over).

While fun in principle, a game like that relies heavily on massive content and good implementation. If not there, the game will quickly become boring.

Why does GTA work so well? Since it's a goofy setting with fast cars and large living cities. Doing it in prehistoric times will work less well I'm afraid. There is less things to interact with, vehicles/animals are slow and not powerful, roads are non-existant so travel is more careful, and mayhem is much harder to achieve (no explosives, no police/swat to hunt you down etc).

While it's doable, i dont think the idea is "brilliant"  

Look at far cry primal. While a quite fun game, it is more geared towards survival than explosive GTA-style fun. Squeezing the GTA-style into a caveman setting seems a bit forced.

@suliman It is not just caveman or ancient era, it is a combination of "eras" all in one time period. It is more like, different regions living in different eras. Cavemen, ancient civs and medieval people living in the early gunpowder age.  There will be cities and sizeable villages with lots of people and things, gameplay will be fast paced, animal transports will be fast, cannons on ships, tribesmen with bows and arrows and swordsmen will be analogous to police hunting you down. So yes, fast pace chaos in this setting will be achieved. Romans built roads, so those, along with dirt tracks will be there.

Maybe no explosions except from canon fire.

Ok fine, but the idea sounds just like "huge game with everything in it, oh and also simply <better> than AAA-studios releasing similar open-world games".

While possible, such a project requires a huge and experienced team. And the "idea" is not what's valuable here, the idea is super broad. It's more of a setting.

Grand open-world ideas are fine to have, but hard to translate to good, actual games. If you plan to realise this project, I really hope you have a big budget and has released some more manageable games already. It rings a "star citizen ambition" warning in my ears

Or as another example: "I'm gonna do a MMORPG. What's the idea? Well it's like WOW but has more of everything. Also, it's better."

Sorry for rant but we see a lot of these threads with grand ideas.

@suliman  What do I need a big budget for? When all I need is a game engine, a laptop a few electricity bills, some free resources from the internet, and a lot of time, and no salary to pay if I'm doing it by myself or have a few volunteers help out?

This is what I've been trying to explain to someone else on this site, but let's stick to the topic.

Ok but when you mension all the assets and gameplay features it doesnt seem feasable to do this alone and with "free resources". Huge open world games are typically one of the hardest genres to produce. If you think you will learn game development from doing it, by all means do, I just say you will probably never have a finished/polished game that is fun for others to play (let alone possible to sell given what you compete against).

On topic: I still don't see why this twist on the open world genre would be particulary fun or even coherent, compared to other open world games.

But I will most probably have a finished, good enough, decent game that is with enjoyable gameplay, ( I do not intend to compete with current gen titles). By the way, you can rapidly create large open worlds with unity and its terrain editor.