I would imagine the design list would be quite long, but in basic terms, to expand a bit on what was already said, I would offer up these points:
First, there are two approaches to designing a game.
A) Create something YOU want to play, and if you then want to put the effort into trying to sell it (or offer it up for free) than that's fine. And …
(B) You start off with the goal already in place to make something that you can sell.
For the first, you can really do anything you like. The design is for yourself. Make something YOU want to play, that YOU enjoy. Have fun. Maybe others will like it as well.
For the second, your need to look at the gaming comminity that you intend to target, see what sells, what does not, and craft your game to include at least some of those things that worked for others. Not saying innovation is not a good thing.
Once you have decided on your approach, then you need to set a scope for your project. Will it be a boxed game with miniatures and cards and tiles? Or will it be a rulebook that people can pay you for the PDF download?
Something that often kills a game in development is that the scope continues to change, so you are trying to design to a moving target. Frustration, boredom, and eventually, the project gets abandoned.
So figure out in general terms what you want, and try and stick to that.
After scope, figure out if you can actually meet that goal with any reasonable chance of success. If the goal is a boxed game with 25 pre-painted minis, a rulebook, cards, dice and all for $50 USD … then start researching companies that print decks of cards and cardstock tiles and make pre-painted minis, and see what those companies would charge you.
You may need to change your scope after getting back some quotes.
And finally, after all of that, if you are still on track, be prepared to spend hundreds (likely thousands) of your spare time hours creating your game, working out all the angles, playtesting, creating a website perhaps so that you can enlist some playtesters so other eyes can try it out.
Be prepared for some to trash your design immediately. Don't take it personally (which is hard not to do). Sound advice is worth more than you can imagine. Some suggestions will lead you to change your game, and it might flow into another direction (which can be a good thing).
But be prepared to sift through the feedback, and learn what is worth taking, and what is not.
Like I said, not a checklist on how to design the actual game, but some advice on how to approach a big project like this.
Good luck.
