It's important to remember that you have to start small. It was extremely badly drawn and the programming was well, 'dodgy' would be a good word. Even though many of the amateur adventures out there are inspired by classic games, it's not always a great idea to base your masterpiece on an established game universe. Fan sequels almost never come up to the standard of the original, and surely the decision to make a sequel should fall on the original designer, not you. There are hundreds of unused ideas out there that could make the game of a lifetime use your own imagination, don't just take advantage of someone else.
Individual developers have their own ways of making their games.
I often start a game without actually considering how it's going to end. I just draw out a handful of rooms and puzzles, code that in, drawing graphics only when I need it, then just draw out another handful, and so on until the ending. To me, the graphics are pretty low-down on the importance scale, as is sound. What really makes a game is a combination of an imaginative story, puzzles that are sufficiently difficult to make me think but not so hard that you end up using every inventory item on everything you find, and well-written dialogue. That's Ben's philosophy, and judging by his games' popularity, it's worked.