After spending many hours combing through hundreds of different io games and one question that always gets asked is, "What makes this io game good?" Generally the best io games are the most fun, but what actually makes a game fun? How do you know if an io game is fun, and most importantly how can you make a fun io game.
The concept of fun is quite difficult to describe. Players will always know when they are having fun, and more importantly they will always know when they are not having fun too. Pleasure is part of fun, but fun isn't simply pleasure. Think about experiences that are pleasurable like taking a hot shower, or relaxing in the sun. It would be strange to call those experiences fun. Good games seem to have a special something that makes their gameplay experience fun. That special something is the element of surprise. Games rely on little bits of randomness and unique, unexpected outcomes to generate surprises for the player. These elements of randomness can be a simple as enemy spawn locations, the contents of loot boxes or the audio that the player hears. This randomness is a critical component of a fun game. We think fun can be best described as the enjoyment of pleasure with surprises sprinkled in.
Think of io games like toys. While toys can be used in games, a toy should be fun to play with on its own. For example, a basketball is fun to interact with on its own. You can play with a basketball without actually playing a game of basketball. This same rule applies to io games. The basic gameplay mechanic of the best io games is fun on its own. Think of slither.io as you're moving around, gathering orbs, or in agar.io as you're quickly trying to absorb as many cells as possible. These are both toy-like mechanics that are fun on their own and separate from the rest of the game.
Another question to ask would be, "If this io game wasn't multiplayer and it didn't have bots, would it still be fun?" For many io games, the answer is no, they are only fun when you have multiple people playing together. If you look at all the most popular io games, they all had game mechanics that were fun to play on their own, the multiplayer aspect is secondary. A potential exception to this rule might be the massively popular battle royale io game surviv.io.
As with many battle royale io games, victory depends solely on being the last player alive, it doesn't really work on its own. However, if you think about it further, battle royale games have multiple phases (if you live long enough). The first phase involves acquiring as many powerful weapons as possible. This involves exploring as many areas of the map as quickly as possible and searching crates and buildings for loot, a process that depends heavily on luck. While you still risk being attacked by other players during this phase, it's still inherently a single-player game mechanic. The multiplayer aspect becomes more important as the game proceeds and more players work to eliminate their opponents.
If you're a developer working on a new io game, hoping to stumble upon the next agar.io, just remember this. Your game has to be fun to play with on its own before you worry about multiplayer. As you're refinining your design, keep asking yourself, which parts are fun? And which parts need to be made more fun?