Open world for 2d games?

So I am wondering how to make my game feel like it is an open world. I want the player to have most of the game under control.

The world I have in mind should have 3 different areas (mountains, wasteland and a forest) surrounding your kingdom.Its is a side-scroller not a top down game.

I had in mind to make my world like in Terraria (it extends to left and right, and doesnt have any additional levels), but I think that that wouldnt work because my world is pretty different, it should be bigger and you would have to return to the kingdom from time to time.

Another thing I thought about was to have a big map. That map would be divided in hexagons. The middle hexagon would be where the kingdom is and you would start from there and continue to the hexagon which is next to the hexagon you are currently in. Hexagons would basically have a purpose of selecting part of the map (or the world) and within each hexagon you would have a 2d level. So I think that might give an open world kinda feel.

What do you have in mind I would love to hear your opinion, or if you know a game that is 2d and has an open world I would like to know which game it is so I could see how did they pull it of.

There are plenty of non-linear 2D platformers, quite like what you've described - the most well-known series are Castlevania and Metroid, which gave rise to the "Metroidvania" genre.

Open world is a pretty arbitrary description for most games, but I think the important thing is to create a world that the player wants to explore - one that's worth exploring in terms of gameplay and risk/reward.

Here are some ideas you can toss around in your head, see if they fit into your game idea:

• Hub-and-spoke map design, or quest hubs. Essentially central points throughout the game to which the player must return. The gameplay could revolve around moving from one hub to the next by completing certain spoke quests (and often getting specific items), and optional spokes provide more gameplay, back-story or special items.
• Indoors and outdoors. Create "depth" in the game world (not just height, although that's also important) by allowing players to enter interiors. These could be shops, NPC homes, secret areas or even new combat areas.
• Smart loading. We don't have the ability to stream data in and out of memory with C2, so I'd suggest breaking up your game areas into manageable chunks. I know this contravenes the classic idea of "open world", but a short loading screen, as long as it's not too frequently encountered, is not a deal-breaker for most reasonable people.
• Side-quests. Use these to give the player a reason to explore an area that they wouldn't normally bother with, or to go in search of secrets. This really encourages players to engage with your lovingly-crafted world far more than to simply get to the end and move on.
• "Key items". Literally, items that act as keys. This is one of the crucial elements of Metroidvania games, and can be seen in many modern titles. Take for example a grappling hook, parachute or jet pack that lets you get to an area that would otherwise be impassable. You can use these to encourage players to return to old areas, which, when combined with the above, could really help to flesh out your world.

WOW that is a pretty long and good explanation, much more than I have hoped for. Thanks a lot this really helped.

Berserky

Personally, I think it is not a very good idea to make an openworld in a side-scroll because you loose sufficient time to make your linear world more interesting.

I memorise an old game which is known as Crusaders of Might and Magic which was released on Playstation 1 and then on PC

The first game is what I really love and it was linear. The interesting fact is that in ps1 NPC'S take decisions through the game, showing that they are alive. In PC there was characters without showing their individual characteristics, they haven't done all (only give a quests) but the world was not linear and game is known as a trash game.

In my opinion, open world is a luxury if you are an indie developer, and if you will concentrate all your attention on open world as the main goal you will collapse

Berserky - Maybe you can take a look at Kongregate's Sandbox Hero.

I'm also interested in making what is sometimes referred to as a "Metroidvania" too! There are definitely a lot of great 2D sidescrollers out there that follow this mold. Check out La-Mulana on Steam. It's currently on sale for 2.99! There are a few others on PC worth checking out too. Rogue Legacy, UnEpic, and Guacamelee are all games of this genre, are on Steam and are controller compatible!

Since you're interested in the genre and want to make your own non linear 2D sidescroller, I recommend you play the games that are considered the originators of the genre too. Those would be Metroid and Castlevania like GeometriX already mentioned. Castlevania: Symphony of the Night on the Playstation 1 and Super Metroid on the Super Nintendo are ranked as two of the greatest games of not only the genre but of all time! I've never gotten a chance to play those specific games but I have played the two Metroid games on the Game Boy Advance and the 3 Castlevania games on the Game Boy Advance. Metroid Fusion and Metroid Zero Mission (a remake of the old school original Metroid on the Nintendo) and Castlevania: Aria of Sorrow (last Castlevania GBA game released) are awesome 2D action adventures! They're considered some of the best games on the GBA.

I wish I had free examples for you but I don't. I know there's a Metroidvania like game called "The Iconoclasts" made using Construct Classic in development. Look that up, free demo exists. Also, look up the Metroid prototype using Construct 2 on YouTube.