Design your game not just for maximum depth, but also for minimum complexity.
This is probably the most important article on this site.
Depth is how strategic and interesting something is.
Your simple game isn't going to be as deep as Through the Ages or Agricola, but most people aren't interested in complex, brain-burner games that take several hours. They want something they can easily understand and be competitive in, the first time they play it.
Your game is not competing with heavier or lighter games. It is competing only with games of the same complexity.
The Depth:Complexity ratio is central to my design process, not an
afterthought or minor matter. Most designers seem to ignore it
entirely.
Think of complexity as a kind of money you can spend. When you spend it, you want to add as much good stuff to your game as possible. Also, you want to "sell" (remove) parts of your game that are complex, but don't add much to the game. That complexity can then be re-spent elsewhere, to add something that adds cool gameplay.
I've played and playtested many games that had all kinds of rules, bits, and procedures, but amounted to nothing more than a simple puzzle. If I want to play a simple puzzle game, I'll play Splendor or Azul, not the same game wrapped up in cumbersome and complex baggage.
It might sound counterintuitive to remove a part of your game that adds positive gameplay, but if it generates enough of a reduction in complexity, it's a good thing, even if you don't re-spend that complexity elsewhere. You need to let your game slide around on the complexity scale. It's okay if it ends up more or less complex than you had intended, if its ratio will be optimised.
My epic pirate adventure ended up as a small, simple pick-up-and-deliver game, with a few dice. It was better for it.
The Depth:Complexity ratio isn't linear. Increased complexity enables exponentially greater gameplay. This means that more complex games and elements must be vastly better than the simpler ones.
You might think your game is fairly simple, for what it is, and that only the necessary stuff is in there. That's almost certainly not the case. Your game is far more complicated than you think it is.
A finished game should be elegant — it should appear childishly simple, and not far from self-explanatory. It should not be covered in text, keywords, icons, and numbers, and look complicated. It should look so simple that people assume it's a very light game.
Coming up with complex things is easy. Coming up with simple things is hard.
Radlands is so elegant that people initially looked at it and said "I don't see what's good here", but then the later reviews were full of astonishment.