The theme of Youtube content is something very important and specific that will later become the hallmark of a YouTuber. To determine a specific theme or what is commonly referred to as a niche is to determine your interest in a topic or theme in advance. Why should it match your interests? Because if you are already interested in a certain niche, you will find it easier to be consistent in that niche.
Here are some niches you might be interested in:

1. Culinary

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This first niche is a niche that is quite popular with Youtube audiences. Usually culinary YouTubers/vloggers make vlogs hunting for food, mukbang or eating large amounts of food, the latest food challenges, and others.
Channel examples: Ria SW, Tanboy, Jessica Jane, and others.

2. Lifestyle

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This next niche discusses lifestyle such as sports, automotive, beauty, or health tips.
Channel examples: Michelle Pan, Bubzbeauty, HijUp, Lovely Planet, and others.

3. Entertainment

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Niche entertainment has content smelling of entertainment such as gaming, music, ASMR (Autonomous Sensory Meridian Response or the sensation of relaxing when watching something), and others
Channel examples: Fadil Jaidi, Jessica Jane, Ricis Official, and others.

4. Travel

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Then is the travel niche that contains content about traveling such as visiting the latest tourist attractions, ethnic places, or extreme tourist locations.
Channel examples: Arief Muhammad, Rachel Goddard, and others.

5. Horror

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Niche horror is also content that is quite in demand by the audience. Usually, this vlog contains guts test activities in haunted places, scary challenges, discussions about horror stories, and other scary things.
Example channels: Jurnalrisa, Sara Wijayanto, Nessie Judge, and others.

6. Religion

(source: personal document)
Next is the religious niche which certainly contains religious content.
Examples of channels: Lampu Islam, Harakah Islamiyah, Audio Dakwah, and others.

7. Education

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The last niche is educational, as the name implies, this niche contains information that educates many people. You can discuss unique facts about the world, chemical formulas, foreign languages, and others.
Channel examples: How Come?, Quipper, Question Mark Rain, Rungguru, and others.
Those are some niches that can be used as your reference in creating your Youtube content. Keep in mind that creativity and perseverance are the main keys to the success of your youtube career. Hope it helps.