Garding levels: Easy, Medium, Hard, etc
Point out when something is difficult.
Higher acceptance of failure difficult levels
Change difficulty based on user performance
Adaptive difficulty. [from Candy Crush]
Based on player skill.
Not visible to the player
Challenging without Blockers
Spend time on Level Desing: [has potential and levels]
Spend time on level design. [If you don't love your game players won't eaither]
Make sure you have enough content.
Mesure everything. Progress of like 100 levels to understand when to introduce new things.
Ex: Dig This has 4700 levels. After 1 year, core users also made content.
Adding meta [after launch]
Building city with money earned
Viable when you have a more user base. In Castle Raid had 3% - 5%.
Consider Gamers as well [after launch]
In general focus on everyone to have maximum impact. But, sometimes gamers spend more time money on your game. So don't ignore them.
Ex: Create multiple play modes: to attract hardcore users and not ignore casual players. Lots of money was spent on these games.
Push notification [after launch]
Adding generic won't help you.
Implement reasonably. Ex: after clearing all levels ask for permission to notify when there are new levels. or in Candy Crush (new lives, spinner, boosters etc)