I'm active in a handful of indie game dev discord servers. With each server, there's always one constant: that one guy who has grandiose plans for a super huge game franchise with multimedia support, merchandising out the wazoo, all that fun jazz. It's laudable to see such high ambitions, but too often these newer devs miss a crucial element to their idea: they need a game to sell.
The second thing these newer devs miss is the need to keep the initial prototypes as simple as possible. Just a proof of concept that mechanic XYZ can even work. You need to ask yourself, "what is the smallest possible product I can make and still call it a game?" This is known as Minimum Viable Product, or MVP for short. Extra Credits did a solid video about it about a decade ago:
MVP is one of those skills that deceptively difficult to master. Cutting out unnecessary fluff without removing critical components requires all the dexterity of a surgeon removing a tumor from a body. Unlike this analogy, you can make a mistake and still salvage the project. But the point still stands. You need to ask yourself, "what can I remove and still have a game?" Going full Checkhov's Gun is encouraged here. Keep it spartan.
Whenever I start a project, I start as small as I can. Take, for example, this card game that I made called G.E.M.S.:
It's a Rummy style game with a little bit of a twist to it, meant to be played over a lunch break. There are different colored gems, which you arrange in runs like the picture above, or a set of five copies of the same gem. To test the idea, I grabbed a bunch of pokemon basic energy cards, and played a few hands with my folks. Just to see if the core concept worked. It did, and even though my mom said it needed more interactivity, the underlying concept was there. Before launching, I added three more card types:
I added Mimic gems that copy another card in your hand. Useful for sets, but not for runs. I also added cards that make everyone pass one of their cards in hand around the table. This adds a bit of interactivity and just the right amount of chaos. It also gives players enough agency to slow down their opponents. Last thing we need is everyone racing and not doing anything to others.
So for G.E.M.S., the MVP is the five colored gems, with nothing else. That's all I needed to test the game and see to it that the core concepts work. All the other mechanics can come later.
When you start your next project, don't be afraid to go a little spartan when you start out. You don't need anything fancy, as long as you can test the mechanics that you need.