Although this post belongs more in the “Game Design” forum, I wanted to comment on your question about what makes an Open World game “Fun”. More accurately, what makes it “Not Fun”.
The problem with very large, open worlds, is the requirements to satisfy them. Otherwise the world is empty, boring, pointless. Otherwise, you’d be better off shrinking the world, game’s scale, or changing the design entirely. So to even justify an open world, you need to meet some content requirements.
Requirements Include:
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AAA Content.
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Reliance on Multiplayer.
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Procedural Generation.
1. AAA Content.
I don’t have to explain the benefits of #1 (Just look at Bethesda games) or the drawbacks (Just look at AAA costs).
2. Reliance on Multiplayer.
Although this requires significantly less work, it comes with significant drawbacks.
This could include a game that focuses on Player-Generated Content, but I assume it’s talking about the recent fad of the Open World Survival Sandbox FFA-PvP genre.
That genre is pretty niche though, nearly always resulting in PvP being a center component. Even if some games are quite popular (although I am skeptical as to how popular they are.) They might be more of a fad than anything else or only draw in certain demographics. We’ll have to see.
The games I’m talking about (and I assume were implied in the OP) are survival games. Day Z, ARK, Hurtworld, Life is Feudal, etc. These games are, for many people, just awful because of their reliance on multiplayer. That is why I categorize them as niche. Their reliance on multiplayer is both their success and their failure. Although I believe it’s the latter more than the former, but at the same time… that is what being an Indie is about. Catering to that niche, because you don’t have the money to “not fail” for the many players, so you acknowledge failing for the many and focus on success with the few.
Otherwise, if you’re talking about a shooter game, a large open world is an even bigger drawback as you’re often better off going with designs that focus on large maps (Battlefield series) or superior gameplay in much smaller maps and gameplay that caters to quick action (FPS games in general).
Why niche games bring in non-niche gamers.
This is here to explain away how a niche game can give the illusion it is popular because of its niche (Open World Survival) when in reality it’s popular for reasons outside its genre (ex. Ark being more popular because of Dinosaurs. DINOSAURS!)
Now, within any niche will be games that break that niche. People who aren’t fans of the genre, but like that game but not all the others just like it. This is because beggars can’t be choosers. As there are so many games these days, there are NOT many games to choose. The vast majority of games are nearly identical to thousands of games that came before them. They are restricted to the more popular genres, AAA budget requirements, or niche genres that refuse to breakthrough that niche (ex. a roguelike genre where the idea of removing permadeath or RNG is ludicrous, so it stays in no matter what, preventing innovation).
An example of what I mean by beggars can’t be choosers is how players don’t just pick games based on their gameplay, but also many other factors like theme or art quality.
- “I want to play a game with Dinosaurs in it!”
- “That game shouldn’t be one of those stupid FFA PvP Survival games.”
- “It needs to be a ‘real game’ so none of those lame 2D mobile games or pixel art stuff. No 2D period. I like 3D games.”
- “Hmm there’s only 3 games left to choose? Oh, that one has horribly ugly art, overwhelming negative reviews, and the other is still in Early Access?”
- “I guess I’ll have to go play Primal Carnage. Oh damn, the servers are all dead.”
There goes that player’s “Vast Ocean of Choice”. So what happens next? They either don’t get what they want, or “beggars can’t be choosers.” I guess I will play ARK, despite how I think 3/5 main design components are horrendously bad. “Yea, the game is good, but it would be GREAT if they made this/that an option or removed it.”
It’s even worse when you’re a gaming veteran who has already played the hell out of “The Classics”. Try to find me a game where you have incredibly depth, an interesting theme, fun gameplay (good reviews), that is a SIM game where you manage a team of superheroes. No wait, just find me a SIM game where you manage superheroes. Damn. Just find me a superhero game where I can create my own superhero/powers without it being a MMO or 3D third person MMO-gameplay game. Anything not Marvel/DC? Okay… how about a game like Neo Scavenger, but isn’t Neo Scavenger since I already played it to hell? Oh damn, nothing? 
3. Procedural Generation.
This requires less work than #1, but once again comes with its own drawbacks. Primarily, the content filling the game is lower quality compared to hand-crafted content.
I am not really aware of all that many open world / large map games that have a lot of procedural generation.
Minecraft & Voxel-Clones come to mind though. After all, Minecraft is an incredibly simple game with a huge open world. However, look at that world. Outside the Procedural Generated Blocks/Environment, what fills it? Next to nothing. Not much to the game outside of the procedural block-world and extreme focus on player-generated content / reliance on multiplayer.
It’s an example of a combination of #3 and #2.
A World Too Big
Vanguard: Saga of Heroes was a great example of a game that had a world that was just plain too big. It was quite an amazing world, and unfortunately a great game that never stood a chance due to technical problems. However, the large world was a huge weakness as much as (arguably more than) its strength.
Even when the servers were filled to the brim with players, you rarely saw anyone outside of central hubs. You could roam for what felt like forever, only seeing 0-3 players along the way. Although popular content was densely population, the rest of the world was barren.
Even the NPC’s felt barren. So you had this huge beautiful, hand-crafted world. They filled it with tons of NPC’s and tons of Players…and it felt empty. So empty, it suffered for it.
A Combo of All Three?
What I’d be most interested in a game (especially one with a world that felt as large as Vanguard) that tries all three methods to fill the void.
- High level of Developer-Crafted Content. NPC’s, Landmarks, Dungeons, Towns, Cities, Animals, Monsters, etc.
- A high reliance on Multiplayer. A world full of thousands of players. Not just that though, but using the Players as a form of content generation. Player towns, player involvement in cities, player dungeons.
- Procedural Generation / Artificial Intelligence. Not as the core, but as the icing on the cake. NPC’s with intelligence to give life to cities. Animals which appear from forest foliage, hand-crafted/procedural events with bits of story (dragon flying down to greet you, soldier battle appearing in the distance, finding random artifact, etc.)
I think with a combination of Player Content + Developer Content + Procedurally Generated Content, the world will feel truly alive. And why not? Player Content is free after you’ve made the tools/systems. Developer content is typical stuff. Procedural Content is more, but generates a ton of content for less effort.
Is there even any evidence to suggest “Open World” games are more common these days?
And by this, I mean relative to the total number of games being released. If the total number is higher, which it is, of course there will be more Open World games. However, has the % of total games being “Open World” changed? I’m skeptical.
I personally don’t notice any more “Open World” games than there used to be. Not many outside of specific genres, anyway. What indies are copying Bethesda? None that I know of (how could they?). So all that’s left are what? Open World Survival games? I don’t think that’s all that serious.
Instead, I think it’s an uber-popular fad composed of games that are in fact pretty damn crappy. (ex. Day Z griefer games). However, enough people “make it fun” via the multiplayer / fad aspect. I guarantee these games also have an extremely low average age. Mostly filled with teenagers still in grade school or college students with too much free time on their hand. After all, most adults don’t have the time to spend 8 hours grinding to get griefed by another player and lose all progress.
There are just a lot of games being released. The flood of greenlight games and the sudden urge for indies to clone one another’s games following the success of some horrible (but popular) mod isn’t a sign that the entire industry is changing toward “Open World” games as a norm.
MMORPG’s have always had open worlds, since their inception. Nothing new there. Same for some RPG’s (Fallout 1 & 2). Even space games that have never been cloned (Starflight) had open worlds.
I’d argue you see less of them now than before. Unless you don’t consider games like Ultima open world. (They sure feel like open world games. Tons to do, tons to explore, and when I was a kid playing Ultima Exodus, you could do things in any order you wanted. Starflight was so amazing, I didn’t ever even do the main story.)