1. Time travel small openworld stealth game:
Imagine a small town were you start out and suddenly you can travel in time like 30 years ( ;) ) and all of your actions have real impact. You travel back, plant a tree, you travel back to the present and its here.
There might be a big diamond coming to your small town as an exhibition and you want to steal it. You can steal it by building a tunnel in the past or other things like starting to work there and copy a key. Or you could become the towns key maker and wait until the museum wants you to copy the key. Or you could become the towns security system expert and actually sell it them.
Problems: When you can travel in time, you are rich anyway. I haven't thought about it for a while what further implications it have but i do remember an nvidia demo were you saw a car age (like it becomes super rusty while watching the video).
The complexity comes from all the implications you need to take care of. therefore a small town.
2. A story line clicker game (spoiler alert! ;)):
Style: 2d pixel iso. You want to become rich and in this world clicking the mouse is how you earn your 'clicks'. You start out small in your kids room. Sitting there clicking (player has to do it manually). After a while you are allowed to move to your parents garage, you get a desk, you can now click faster. Than you hire some friends.
The transition is basically: your kids room, garage, small office, big office. When you start owning an office, your character sits in front / at the top and looks down to office talbes and the clickers.
Over time you can expand, you can order overtime, you need to hire new staff. If you burn them out you have to hire activly more and faster.
As a side quest you could persue a romantic relationship.
The game is over when your character dies. Your character dies of an heart attack in the rage of 60-90 years depending on what side quests you do and then when you had your heart attack, a high score is calculated additionally to all of clisk you got.
The twist: you can also persue a romantic relationship and if you do that and you spend time with your partner (like in mini events) you earn way less clicks but 1. you hit a higher age like 80-100 and 2. surprise: your highscore gets an additional happines multiplyier which will always be higher than your highscore without a partner in life. This also unlocks a hidden achievement and the happines mode / display and only after you went this route the happines factor is shown in the highscore calculation.
Basically the game should motivate you to be super aggressive first: Lots of overtime, killing your employees and using drugs and rehiring constantly for the persuede for the highest score and after your second playthrough and achieving a specific highscore you get hints that it might be better to be happy.