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After this step you want to really get stuck into your game mechanics. Work out every kink and chop and change them until they are right. While this is happening all that research you did will be informing them and you will be making adjustments to your concept as you go. Once you are happy with the mechanics take your concept and lay it out. Work out the key story beats, how many characters you will need and what they are like. Think about what happens before, during and after your game. How the world of the game works. How the main character moves. The basic structure of the levels. All that stuff.
Now you should have a relatively formed version of your design before you. The next thing to do is to go through your research again and do some more if you think you need to. Then look at your mechanics again. Chuck the ones that don’t look right and think up a couple more. Then go back to your expanded concept and story and chuck out all the deadweight. If you think any part of your game’s story, world, characters, etc. is to confusing then it will be completely baffling to everyone else who is going to read your design.
After all of this you will have the basis of a game design. From here you can start fleshing it out into a fully fledged Game Design Document (which is another post’s worth of stuff in itself). While you are working on the GDD for your game you should periodically go back and re-read/tweak your concept and mechanics while looking through your research and expanding upon it when necessary. It is a very repetitive process but trust me it works. Every time you go back you will change something, throw something out or add something new altogether.
That’s how I take my initial ideas and work at them until they are at a point were I can look at them and see if it is a good idea worth a full GDD or a bad idea that really doesn’t deserve to be made. There are a last couple of points I want to make. The first is be overly harsh with yourself, the harder you critique yourself the better your ideas. The second is talk about your ideas with people who you can trust in game design, for me this is my fellow students, they can point out things your are missing and help with your overall design.
I love game design, but damn it is a lot of work!
im currently using the story to base my designs but sometimes a small game mechanic pops in my head and that becomes the start of something so yeah and research has always being interesting and very useful alot better to understand something on a specific topic before putting it into a GDD p.s *ROBOT TURRET VOICE*ARE YOU STILL THERE?*
Here's how I see it.
Create an amazing and compelling idea, evolve and develop the concept, turn it into a game. Develop the game and release it with not too much content but enough to warrant the price tag and use the KISS method. (Keep it simple stupid.) Make it memorable but don't shove it in people's faces or try and fight other big franchises, that won't get you anywhere.
Then take the second step, formulate a way to improve the experience, expand the story and keep the player wanting more. Add a little more content but not too much and try new things to bring in more people. Make it fun and enjoyable but make them know it could be better. Show people this could really be a contender.
Final step. Wrap everything up in the most awesome way possible, give them something to scream about and leave them wanting more. Add a tonne of content to keep them playing and perfect your game. From here you can do whatever you want.
If you are looking at sequential games like you are Dan I would say never think up your game as a trilogy. Entertainment media has an obsession with trilogies and in games it is not needed. You should be looking at what makes the game you are working on at the time the best it can be (particularly with new IPs). Never leave anything out because it would make a good "new feature" in the sequel.
Also we are currently at a point in the industry where publisher's don't really want trilogies. They want franchises. Eight games all baring the same banner is going to make more money for a company than two separate sets of three games. You can work on the beginning, middle, end structure of a trilogy with the first three games but always leave a big enough space for another sequel.
Even if you see the game as only having one release always leave room for someone else to come in and make that sequel
Me and only me alone work on my I.P.s! xD