Elanso

1. It seems natural to include a scene in the restaurant, Milliways. Could be a bit of fun: strange parties, unctuous compere, self-introducing food. Perhaps there's an object there that you need to get. (It could be a SPORK, a spoon with sort of forky tines on the end. Or would that be a FOON?) It could be a vehicle from the car park -- Marvin has the keys. If you manage to re-enter Milliways at another time (oops! on another occasion), you will not meet yourself, "because of the embarrassment that usually causes." What about a visit to the Big Bang Burger Bar?

2. Given point 1, you must have a means (or several meanses) of time travel. In fact time travel instead of space travel could be the primary method of changing scene. In the original, the party got to Milliways by accident: in the radio version, a "hyperspatial field generator" overheated; in the book version, Zaphod's great-granddaddy screwed up the works of Eddie, the Heart of Gold computer. Maybe your trip to Milliways would require info from an anti-piracy device in the game package. Once at the restaurant, you can steal a timeship and go anywhen you want.

3. Given point 2, it seems natural for the "best ending" of the game to be your arrival on Earth before it's destroyed, which is the ending of both the first radio series and the second (namesake) book. The original route to this ending was an accidental landing on Golgafrincham Ark B, with its cargo of telephone sanitizers, marketing consultants, etc. (the ancestors of Earth's humans!). I rather like this bit, and hope we can work it into the game.

4. Okay, so what about the beginning of the game? The easy answer: take up the story where the "Hitchhiker's" game left off, namely the arrival on Magrathea. But in the original this arrival is followed by a travelogue of Magrathea and a flashback to the Deep Thought v. philosophers' union story (including the introduction of the "42" joke) and the joke about the true nature of mice. All funny bits, but I have a hard time envisioning how they can be made into interesting interactive versions. Perhaps you could time-travel to Deep Thought and interact with it yourself. The Magrathean catalog of planets on Sens-O-Tape could be useful.