This theme is probably gonna be pretty tough! We are talking about some big genres. So it will be extremely important to make an extremely simple idea. Here are some examples for what would be truly simple:

1) A city builder that has no resource gathering and only a few buildings (like Islanders, but simpler) https://store.steampowered.com/app/1046030/ISLANDERS/

2) A survival game where you're only gather one type of plant, with a twist

3) An RPG but it's only turn-based dueling, or only dialogue with one npc, or only character creation

I hope this helps you get some ideas! Get creative and keep it super simple! FOCUS ON POLISHING!