Here's some ideas and samples for mechanics :)
What would a time limit do?
What would energy economy do?
What would a limited amount of a core item do?
Make unlimited into limited
Inventory and its size
Size of space, introduce expanding
Risk & Reward
Chaser becomes the chased one
Gain some, lose some
Booster power-up to change balance
Bigger size, Smaller size, More power, More speed, Bigger Jumps
Tesla – shoots around randomly
Bullet time, Double Time, Extra Time
Health pack, Extra life
Key & Lock, Levers
Do this once you have the core gameplay ready do this: Draw it on paper
Create paper power-ups, level objects, characters, npc
Play the game
–You can play it turn based with dice
Does it work? Is it fun?
You can also draw any other screen to test it
Out of the 100 games I've worked with the score system surprised me suddenly, During the project I considered it obvious - until we really had to do it - and it came obvious quite soon that what I had in mind just didn't work.
Think what will be your scoring system in the game!
How will a good player do?
How will a bad player do?
Is there variation? Will best players always get like 200 - if so, create variation and ways for players to never get the same
What do you get from ”kills”?
Does time affect? How?
What do you do with score? Is there a shop?
What is the high score?
How will score work between different game modes?
Should I have leaderboards?
Create the score system and calculate how player will do!
Introduce combos, double points power-ups
Have score system contribute to replay value
Sometimes a limit like "survive" or "make it in time" can be a much clearer objective than "Get 10320 points"
A simple variation to Tetris is a game like 1010. Here we only change how you get the pieces. Instead of them dropping, you get them to the bottom and from there you can place them anywhere.
What is your variation?
What is the Risk & Reward in Tetris?
The risk is every time you decide to turn a piece to a certain position, you might run out of space. The reward will be fitting the piece optimally and maybe removing a row.