1.Easy to understand.
The first thing that game need is easy to understand the way to play. If your game has a tutorial. You must make it shortly until the game starts.
Usually, only 2 things to guide players are Where does the coin come from and How to make more.

2. Coin mechanism
Almost idle game have a core loop. Earn coin, upgrade or explore and earn more than.
A good idle game has many ways to make a player earn coins and use them.
Amount of coin based on the ways that game design. For example, click as fast as they can, this way is better than the game run automatic.

This is one of the most important functions. The game will be boring if not contain it and make sure the player must use it. They will make your game value. Using your graphic to attract the player. For example on oil exploited game, the factory level 1 has one lever, but level 2 has three levers. The player will play a long time If your game has good logic about upgrade.

The game must have a percent balance among earn and spend.
Each item, upgrade, unit need the own value. Make sure they are not wasteful.
When players need to upgrade or make something, the coin amount will be less a bit or have a limit to make players try to make more coins.
We will make the player feel comfortable, not drive them to follow your game script.

5. Explore
After they played for a while, they will get a habit of the game system.
We will increase the difficulty about earn coin or show they many options about spent coin to make them stronger or more profit.
Example: open another land, company, or add a new unit to which game systems.

6. Offline time
The second important thing about idle game.
IDLE game does not punish players like the Resources Management game. When they come back, your resources exhausted, production stagnated,they are robbed, invaded while offline.
When designing an IDLE game, we need a softer mechanism, the game system will be run and make a profit while offline.
When login to game, they collect that profit. They spend and upgrade before the coin will be limit again.
The key of the IDLE game is “If they don’t play, they still have money. They play more and got more money. They get big money offline after a while”.
So Idle Game has an extremely high return to play rate.

7. Simulation and interaction
Players will feel much closer and more excited if your game simulates an activity or a mechanism that exists in real life.
If possible, make the objects in your game as interactive as possible. And at the same time, any interactions bring some value. Example: Drag to speed up the production of the conveyor belt, Tap continuously to speed up the production by the worker, etc.
There is no need for tutorials for these interactive sections, but leave it as a signal to keep players excited as they figure it out.

8. Restart location
The restart mechanism is a function to consider when it comes to games because it has many aspects.
Restart here is not about replaying the entire game in the usual way, but just restarting certain areas.
This solution applies when Game Designers need a mechanism to reuse the amount of data in a certain area.
Usually, after restarting, a multiplier is needed to increase the amount of profits, to reduce the replay time. This is considered as a reward with other shows (raising stars, giving medals, unlock achievements, etc).