Since nobody out there seems to have a script or an idea for a script or a rumor of plans for an idea for a script for a sequel to _The Hitchhiker's Guide to the Galaxy_ movie, I'll throw my own idea out into the aether. Copying the book wouldn't make for a good movie, I don't think, since for a good bit of the time our heroes Arthur and Ford are in Zaphod's pocket and completely out of the action. So my concept would include action bits from the radio series that were left out of the book.
We'd start where the first movie left off, with Arthur, Ford, Trillian, and Marvin on the Heart of Gold, heading for Milliways. And let's put Zaphod and the Vice-President on the ship, too, because they never quite said on what ship they left Magrathea (although the big Z did say "just you and me"). So everyone is arguing about how to best make the time jump to the restaurant or even should they go get Zaphod's other head from Mr Sneeze first. Basically, it comes down to Trillian saying they need to get Zaphod back to "normal" and the Veep (who is a woman, by the way) who likes el Presidente just the way he is and therefore wants to go straight to Milliways.
Arthur gets bored and goes looking for tea. And here's where the real story is. No matter the pretty pictures, you need a good character-driven storyline. In HHGG, it was Arthur and Trillian falling in love. In REU, it sadly has to be them falling out of love (or else the further stories are impossible). Trillian wants a man who's an explorer, and Arthur ain't it. He's much more interested in finding his cuppa than seeing what the end of the universe looks like. Throughout this movie, Trillian has to gradually realize who Arthur is and that he won't change.
Back on the bridge, Zaphod takes Trillian's side in the argument. In the galley, Arthur yells at the Nutrimat machine and manages to tie up all the ship's computers with the question, "And do you know *why* I want boiled leaves in water?" So when the Veep kidnaps Trillian (out of jealousy of Zaphod's affection for Trill) in the solitary red escape capsule, they are unable to pursue.
And then the Veep sends the Vogons to destroy the Heart of Gold. Ford, Zaphod, and Arthur hold a seance to channel Zaphod's great-great-grandfather, who gets them out of the mess because he doesn't want them in heaven with him.
So they improb out of there and wind up in a huge marble cave thirteen miles above the surface of a planet. Thus follows the bit from the radio series on Brontitall with the giant statue of Arthur and the smelly bird people and much cliff-hanging and cliff-falling, but in this version it would be on the Frogstar. Perhaps Arthur could even learn to fly (temporarily) on his way down to being caught by a bird. Seeing a fifteen-mile-high statue of yourself is likely to make anyone forget about falling.
Meanwhile, Trillian is learning the truth about her semi-alien ancestry (whatever that might be) when the Veep does some DNA testing on her. Oh, and by the way, the Veep is taking Trillian to the Frogstar to put her into the Total Perspective Vortex.
Ford and Zaphod also fall out of the giant cup ("Belgium!") and land on the back of a passing bird ("Look, this is utterly ludicrous."). Marvin falls out of the cup and crashes a mile deep into the subterranean corridors of the Vortex complex. After a talk with the bird people, Arthur travels down to the ground and starts getting shot at by Frogstar robots. Lintilla the escort-agency clone archeologist rescues him. She is, of course, excruciatingly beautiful (which puts further strain on the Arthur-Trillian relationship).
Trillian, who is quite clever, escapes from the Veep and her Galactic police.
After a bit about shoes, the Frogstar robots find Arthur and the three Lintillas. And then Marvin finds them. And then Poodoo, the Priest, and three anti-clone Allitnils find them. ("Excuse me, but I have three impromptu weddings breaking out behind me!") One of the Allitnils is shot, but the other two cancel out two of the Lintillas. Poodoo and the Priest escape, but Arthur, Lintilla, and Marvin are captured.
Frogstar robots capture Ford and Zaphod as well. So now we have Arthur, Ford, Zaphod, Marvin, and Lintilla in captivity -- with the Veep standing beside the Frogstar robots. But she wants Zaphod to love her, so when they put Zaphod in the Total Perspective Vortex, the Veep fiddles with the knobs so it goes easy on him and tells him that he's the most important person in the universe ("something he hitherto only suspected").
Trillian rescues them. Zaphod acts heroic while Arthur cowers. They outrun Marvin. Shooty and Bang Bang (Galactic police) corner our heroes behind a computer bank which proceeds to explode and send them hurtling through time to Milliways, the Restaurant at the End of the Universe. After realizing they're not dead, they finally get a chance to relax and have a decent meal, or at least a good drink. Ford talks to Hotblack, they meet the meat, Marvin calls from the carpark, Zarquon appears, the universe goes "foom." In the carpark, Ford and Zaphod want to steal a cool black ship, but Arthur comments on the "Disaster Area" logo and they decide to steal the next one in line, which happens to belong to the shape-changing Haggunenons. Trillian somehow manages to ditch Lintilla.
Of course, things go badly (with much cinematic shape-changing), and our heroes are forced to use the escape capsules. Trillian goes with Zaphod instead of with Arthur and Ford, which totally bums out Arthur. And then Arthur and Ford see that there was not, in fact, a second capsule and thus Zaphod, Trillian, and Marvin are all dead (or so they assume).
Arthur pushes the big red button (hoping it's "reverse") and they wind up with the Golgafrinchams on prehistoric Earth, whereupon they discover that the whole Ultimate-Question-of-Life-the-Universe-and-Everything computer program got messed up by their arrival. And they play Scrabble with cavemen. ("What do you get if you multiply six by nine?") Ford consoles Arthur about Trillian. Cue Louis Armstrong. Not a happy ending, but perhaps it can be sanguine.
And Zaphod doesn't get his head back. But I suppose he would, off-screen, before the start of _Life, the Universe, and Everything_.