First, the Eighth Circle of Design Hell:
Flattery and Imitation
You can take a game and retheme it. This is as simple as making Settlers of Catan into a game set on a different planet, making a local version of Monopoly, redoing a Vietnam war game as a Space Marines vs Tyranids warhammer 40k war game... you name it. This is the easiest to do and generally only requires theming components, not changing mechanics. It can also be a disservice to both the new and old theme such as in the case of the Vietnam to Warhammer 40k pipeline. Tread lightly, don't make light of the plight of others. Repsect your subject.
But it's a valid way to "create" a game, and you should keep this in mind for when you're getting started. It's a way to get comfortable with the language (metaphorically speaking) of design.
Second, the Sixth Circle of Design Hell:
Heresy and Expansion
Expanding a game is a really good way to come to grips with both how all the mechanics of a game work together, and how one or two specific mechanics, systems, or ideas can be done. This would be adding different islands to Catan (Seafarers of Catan) or maybe adding some more complexity to armies and city building (Knights and Cities). It could be expanding a historical game to a related time period, or just grafting a combat mechanic on to Monopoly because you're a sadist and why we can't have nice things.
Third, the Fourth Circle of Design Hell:
Greed and Stealing Everything That Isn't Nailed Down
Taking a game that you like and using its system and mechanics to support a different subject is a lot of designers first foray into game design. I'm part of staff for the COIN Series Player's Club Discord and people who started as fans of that series have created volumes set during: the Bakumatsu (1853 - 1867 Japan), the Onin War (1467 - 1477 Japan), modern day Somalia, the British Emergencies (1940s - 1960s: Malaya, Kenya, Palestine, and Cyprus) among many others. If you find a system and you think to yourself "Y'know... that could underlay the model for this other thing pretty well..." Do it. Give it a shot. You'll have to touch every part of the system in one way or another so it's a really effective crash course in game design. Some systems are easier to do this with than others, and fictional subjects are generally easier than historical ones but don't let that stop you. Just treat your subject with respect.
Fourth, The First Circle of Design Hell:
Limbo and the Virtuous Nerds
Make a whole ass game from scratch without directly borrowing significant pieces of it from the giants who shoulders you would otherwise be standing on. I have no real advice here - this is really hard to do in a way that's A) authentic and B) good. Good luck, god speed if you go this route.
What I'll be Demonstrating
I'm going to show the Fourth Circle of Design Hell, by stealing a bunch of neat things in service of the idea I want to model.
In case you're wondering why I'm describing design as hell, think of it as development hell/production hell and not in terms of being hell to go through. It just (mostly) takes a lot longer to design things than you think it will. You'll be done with 90% of it pretty quickly, but that last 10% takes forever. This is just the way things go.