TL;DR
Because people tend to find a new feature and a new game mode, it’s best to adapt to such demands.
Dynamic/Fancy game
You know that more than half of the player base are underage, who are below 18. Even though they are not eligible for playing modd.io games legally, they are nothing short of the main target of your game. Imagine when you were a children, and most of them are basically fond of extreme, fancy and instinctive things. Although some of them like puzzles or difficult parkours, they are niches. So, make your game include dynamic movements and try to use fancy colors and animations.
Constantly update your game
Although it is not limited to children, they will leave your game once they think it’s not fun anymore. So, you need to constantly update your game to keep it popular. In some cases, there will be a fixed fanbase, but without any updates, they will leave eventually.
Provide unique tactics
It’s important that your game has a difficult aspect to be perfect. It should be easy to understand, but difficult to master. The former will attract new players, and the latter will keep attracting the veterans by pursuing to further improve and making them proud of their skills.
Game building in a game
If your game provides users to create their own levels in your game, they will linger much longer. User-generated content will not only attract new and existing fans but also improve the retention of the user who designed it, and hopefully it will mitigate the work for updates and new features a lot. You can mandate some of the new features to the user and their creativity! It’s a win-win relationship and it is highly recommended. However, because modd.io is also a game-making platform, it provides a limited range of functions, so it might be difficult to create one with high degree of freedom. Nonetheless, Simon Says and Idle Tycoon have been favored by the players for years! (The former has another reason though)