The best thing you can do is draw comparisons to something you're more familiar with, and decide how you would start tackling problems for that. What you'll find is that rules have the universal goal of creating a type of experience. It's your job to decide what that experience is and ensure the rules help that experience come about.

I don't typiccally put much thought into magic systems for my RPs, but I wanted to do something that felt a bit more like a game without actually being a game. I was tired of mages who never ran out of mana, juxtaposed by spell swords who were both weaker and didn't have the mana to cast more than a few meaningful spells. But I was more interested in tempering how people used their magic than deciding the outcome of the encounter. So the system I created allows for quite a bit of freedom. You take "notes" and string them together to make a spell. The more notes you use, the more expensive the spell is. The parameters for one's mana and the spell's cost is very rigid, but the effects and other parameters are fluid enough that the game aspects never overshadow story telling ones. It just just what I want by allowing magic a great deal of freedom while keeping everyone's mana in check.

If you're looking for a gamey kind of experience, I would consider looking at some board games that might give you the feel you're looking for. Monopoly and Catan are probably worth looking into, but there are less common ones that might be of more interest to you. Sheriff of Nottingham is literally a game where you try to get goods (and sometimes contraband) past the Sheriff of Nottingham. Look at how turns work and see how that corrilates to posting order and such. While monopoly is easy enough to work out, Sheriff of Nottingham requires some back and forth and would be better suited for speed post RPs or collaborative posters. From there, you can scale back or add rules as you see fit.