I want a good Unknown Open world game, can you suggest something please? :D

I have a few answers.

If you are willing to sacrifice some of the open world bits in favor of deeper character systems, Shadowrun Returns is a must-play. There's an absurd amount of ways to build your character, and just about all of them are viable. The follow ups, Shadowrun: Dragonfall - Director's Cut and Shadowrun: Hong Kong are also excellent.

A somewhat obvious choice, but if you haven't done so yet, Fallout: New Vegas truly has some of the best DLC around. Old World Blues specifically stands out, but I wouldn't hesitate to suggest a new playthrough just to check out any of the content you haven't seen before. Make sure you have Wild Wasteland enabled.

If the RPG stuff isn't for you, there aren't as many obscure options. It's more like "hey, did you not play Saints Row: The Third or Saints Row IV yet? Fix that."

I've had a lot of fun with Grim Dawn lately. It's by some of the team that worked on Titan Quest. It plays like a pretty typical dungeon crawler, but like in Titan Quest, you get to pick a second class. The result is weird combinations like a rogue explosives expert, or a mage that can use armor and shields just as well as a warrior.

The S.T.A.L.K.E.R games are a bit RPG-y, but if you like weird stuff, fun open world experiences, and getting scared shitless, they are worth playing. I recommend starting on "Master" difficulty. There's a weird thing with the settings where the scaling doesn't work quite right, so while Master is hard, you drop enemies just as fast they can drop you. It makes for a really intense, but fun, experience. Normal is just kind of annoying, and Hard just has you get killed quickly without a chance to fight back.

I had a good time with S.T.A.L.K.E.R.: Shadow of Chernobyl, if you decide to go that route.

I hope this is of some use. Sorry they aren't exactly "unknown", but there isn't a lot out there that is such while still being, you know, good.

I think GUN is a fairly underrated open world game. It's pretty short and maybe a bit too easy but very solid overall.

EDIT: An open world title that could be considered generally unknown (forgotten is probably more accurate) is American McGee's Scrapland. Haven't played it myself but read some positive things about it.

Mad Max.

Painfully underrated game. Divorce it from Fury Road and you'll likely love it, compare it to Fury Road and you will sabotage your enjoyment of one of the best games of 2015.

Dragon's Dogma: Dark Arisen is very good if you don't mind walking around a LOT.

Kingdoms of Amalur: Reckoning. Don't get bogged down in all the side content. Do the major optional storylines (Mages Guild, House of Ballads, etc.) but otherwise just push through the main game. Vibrant world, great combat system with a surprising amount of variety and customization, but for all that is good and holy don't do everything in the game. Suppress any and all completionist instincts you may have.

If you want fantasy, the Risen and Gothic games are alright.

For military shooter stuff, there's the old Operation Flashpoint games.

If you want something truly forgotten (maybe for good reasons? I don't remember if the bugs were ever fixed) - check out Boiling Point.

Bethesda made some rad open world Terminator games in the early 90's too.

If you want space combat, there's a lot of open world ones. Obviously the classic Elite series, but also Privateer,Freelancer, Allegiance, Independence War 2, the Parkan series, the free Pioneer and Oolite games and Dark Star One.

S.T.A.L.K.E.R..

Shadow of Chernobyl is my favorite but Call of Pripyat is the most approachable.

@fisk0: Good call with the Operation Flashpoint mention. Cold War Crisis and the two expansions are such good tactical shooters, and the open world aspect was, and still is a fantastic feature. I regularly go back and play them just because they are such unique experiences.

I don't care what anyone says, that Scarface game was alright.

@bigdaveischeap:

Word! The Mad Max is one of my favorite games.

Sleeping Dogs

The STALKER titles.

Fisko brought up a lot of good space trading titles.

Mount and Blade. It took a bit for things to click, as there are a lot of weird fiddly RPG bits that'd make a fan of German board games feel right at home, but once you get it, it's super addictive.