An endless game consists of the same gameplay mechanics repeated over and over. You have to keep doing mostly the same things as long as you can.
So it is quite easy to make players bored of your game. What are some concepts that one could implement into the game design to keep the player engaged?
To be more specific, my game is an endless tapping game where you have to tap rockets to make them explode into fireworks, and sometimes on falling meteors too.
Provide visible goals for the player to achieve. For example:
One important aspect is to make the player aware of what goals there are and what they need to do to achieve them.
I would suggest adding a mechanic that encourages emergent gameplay. Often emergent gameplay can be introduced by adding physics into your game.
For example, if a rocket explodes, it could launch fragments of itself across the screen, and if those come into contact with another rocket, it would also explode. Then the player could try to get the biggest chain-reaction they can, which wouldn't be a defined goal of the game, but rather one the player comes up with.
That may not be the best example, but I hope it helped.
You need to have something that keeps changing. Interest will be replaced by boredom if it is the exact same game over and over. But it doesn't have to be.
Many classic games are the same game, but you play them with different people. Chess or Go, for example, have zero randomness except for the people playing. That makes it interesting to keep playing the game.
Other games have elements of randomness that keep the game fresh. Card games or dice games. That means games never exactly repeat and while if you play long enough you will spot patterns, you never actually play the exact same game twice.
A third category is ongoing storytelling. If your game develops over time, even if the core game mechanics stay the same, it can provide interest. Real world wars (seen as games following James P. Carse definition) are not interesting because weapons change, but because they are part of the history of the world. Many roleplaying games fall into this category. The content can be created by players.
A fourth group is procedural content generation. You can make the game endless not just in time, but also in content. If there is always one more world to discover, one more level to explore. This is similar to randomness above.
What I found the strongest factor is human interaction. If you have a multiplayer game, the fact that players constantly adapt to each other, providing a constantly changing environment, you can draw people in a lot. My most successful game, which has been running without interruption for almost 20 years now, mixes the history and interaction elements and is still interesting to players (some of whom have been playing for more than a decade).
In your tapping game, obviously the aspects requiring other people are out. I assume that you already have some randomness, but this aspect is not strong enough to provide gameplay in itself the way that it does in card games (where playing good with the hand you were dealt is often exactly the challenge).
I would advise to play with randomness and procedural content. You can make levels more chaotic as the game progresses so that the skill a player requires slowly changes from prediction to reaction.
By nature, your game is probably most close to a card game. In this class, the successful games all have a very good balance between skill and randomness. While you get a random hand, the game is balanced so that a good player can win with almost any hand, while a bad player can absolutely lose with even the best hand. There are often also multiple ways to win so you can choose a different strategy based on your hand.
Your game may simply be too simple to have these elements that a game requires to be eternally interesting. You may have to add such elements.
A user will not play a game where they do the same thing over and over. It will get boring and they will leave. As such, you need to make it feel like they're doing something new, even if the underlying mechanic doesn't change.
The ultimate example of this is probably Candy Crush. The mechanics introduced in level 1 are basically the same mechanics you use in level 1800. So what changed? The big thing that changes is the difficulty. You have to do bigger and better chains and plan further ahead in order to win.
Another ultimate example of this would be the famous games of Chess and Go. Both of them introduce 100% of the mechanics in the first week of your playing the game, but those mechanics interweave into endlessly intriguing things which people devote their lives to mastering.
Chess and Go demonstrate one very powerful tool for keeping your game interesting: strategy. If the tactics are simple, but the strategy is vast, then players will keep going back for more.
In the end, the best answer is no gimick. If they feel their life is enriched because they play the game, they'll keep playing it. That is the true art of making a game that lasts. You can put as any gimicks in as you please, but its the users who invent the real reason they play.
Take fantasy football. There's almost nothing to it, mechanically. But it is incredibly popular. It's popular enough that if you search for it, the Wikipedia article on it isn't even on the front page! This popularity is because players feel that they are close to the real action. They create their own purpose for continuing to play the game.
Expanding Philipp's content unlocks category, add powerups/items that change up the gameplay. For example: You may tap to explode a rocket, but what if there was an item that made it so when you tapped, a "tower" would spawn that would help you. That kind of stuff is why Enter the Gungeon and The Binding of Isaac are so popular today
I feel that there are two important factors that would cause a person to keep playing a game like this. Extreme difficulty and quick, visible rewards. The ratio between these two is important to test out on an audience.
If the game is too easy, they'll get bored with it as it doesn't offer any challenge.
If the rewards are slow to come, then they won't be incline to continue because there isn't any gratification. The rewards could be as other have suggested (levels, power ups, achievements, a narrative)
Most of this is from my personal experience playing mobile games, many of which have no end state, are free to play, and rely on frequent restarts to generate ad revenue.
If you are too lazy to make an expansion, a new level or sth else, let the user do it:
Custom levels are always pretty hyped
OR:
Look at Minecraft that Game was released a century ago and is one of the most played games, because they had extra content (produced mostly by the users),
like custom server with different rules, custom games like Survival Games.
In your example you're limited to a 'piano tiles' like game, so you have options like:
reverting screen: Geometry dash does this too
Bosses: needs to be tapped 30 times, or must be encircled
Powerups: more damage, explosion damage, shield, extra heart, etc
Game mechanic change: instead of tapping, you slash now, like Fruit Ninja
