I’m not a game developer, but I’m a big fan of adventure games and let me tell you that making a great game is like writing a book. The most important thing in a game, in my opinion, is the story. All else comes second. Write a game, like you would a good story and use that story as a basis for your adventure. I’ve noticed that a lot of Ken and Roberta’s games use names and concepts from thier own lives. Try drawing from your own experiences, such as trips you’ve been on, the area you live, friends with interesting personalities and see what you come up with.
The second part of your game of course is translating the story into the kind of game you want to end up with. Storyboards help you brainstorm and give you a general idea of how you want your characters to look, and/or act, though if your not the best artist, you might need to find a friend who is good at art or hire someone who’s experienced with creating illistrations.
As for creating the game itself, I can only guess at the options you might have. You could use a system that is already in use in other games or create your own engine from scratch. What I mean by ‘engine’ is basicly the mainframe, or the core that holds the whole game together, a certain format that some or maybe all your games would follow. You notice that a lot of the Kings Quest games followed a similar format, growing steadily more complex as they added more features with each new title.
This is mostly just from observation and I may not be completely accurate, but I hope it helps give you some ideas or generates some other thoughts from visitors.
Good luck with your game.