II/ Mechanics

Now you thought about those important things, in this part I will share with you some rules I analyzed in the gameplay mechanics of most of those games.

Think small
Simple, intuitive and limited gameplay with just a few possible actions (3 or 4 different actions), including a luck coefficient and positive feedback. Fast spaced gameplay, with a small amount of content (minimum 3 zones cuttable into levels), but the final goal needs maximum upgrade and a good mastery of the game: player is accustomed of game over screen.
Because of that, it needs to be the less frustrating and fastest possible to restart from the beggining.

A changing situation
The more the gamer plays, the more he can go farther. This phenomenon is called positive feedback. When he plays, he earn money or XP that he can spent it in upgrades thanks to an evolution/shop system. Note that the price of an upgrade have to increase at the rhythm of more or less the double after each new point bought. At least 2 categories (see the paragraph about it) cut into 4 capacities/item for each. After reaching a level of power, new content unlock: new mobs, bonuses...
You need at least 6 novelty with 2-3 upgrades of them (for exemple a new mob and then the same armoured mob).

Two general categories of upgrades
Active action: Activated by the player, instantaneous or with an effect limited in time, with a limited amount of uses or not. Can have some subtetly like a certain percentage to accomplish a powerful action/critical under conditions.
Exemple of this type of upgrade: "Weapons", "Special Abilities"...

Passive action: Can have a certain percentage of occuring chance under conditions, or unlimited in time. Increase generally luck, earnings or general power (strenght, speed, mana...). Can support active action too, or at contrary affect mobs with a malus.
Exemple of this type of upgrade: "Stats","Chance of bonuses occuring" (like in Burrito Bison)...

Note: Some categories can mix up passive and active actions, as "Magic" that can restore slowly health (passive) and shoot fireballs (active). In fact, it depends how you want to arrange your upgrades, this is just an exemple of general classification.

However, you can ignore or minimize the impact of this part to simplify your gameplay again, but you will have to replace it with a bit more of content.