So to mark the event of his birthday, what Douglas Adams flavoured Doctor Who might inspire a game?
The Pirate Planet is the least famous of his three serials, and the most straightforward adventure as well. Space pirates! But what about this dropped plot thread, mentioned here? “Adams had conceived a drug addiction allegory, about a company which preys on people who fear death by offering machines which can slow time for them -- but at an exorbitant price. The company goes bankrupt, however, leaving one old lady in need of a source of fantastic energy.”
City Of Death is one of the classics (number 8 in the Mighty 200, while TPP is at 109, and on The List provided by Russell T Davies to show Julie Gardner what Doctor Who could achieve) so it’s too well-known to borrow from heavily with classic fan players, but the basic idea of a conspiracy working together across time could easily be used again. Or consider the earlier versions detailed here. Aliens cheating at a big casino... maybe they’re after a particular prize?
Shada is well-known for different reasons. It’s the unfinished classic of Doctor Who, since restored with the Fourth Doctor in bridging narration, the Eighth in animation, and Gareth Roberts in novel adaptation, as well as Adams recycling rather a lot into his Dirk Gently books. This means several versions exist, so you could lift from any and all of them. You could borrow the central MacGuffin of a Time Lord prison - a place isolated in time as well as space, with the most terrible criminals of millennia in there together. See also Timelash!
He also had a hand in Destiny Of The Daleks, so the Movellans are fair game too.
And from his time as script editor, we nearly saw a Gallifreyan political thriller by Christopher Priest and an alien world by John Lloyd of Not The Nine O’ Clock News and a great many things since like currently QI.
You could also quite fairly swipe plot elements from Life, The Universe And Everything, since it was nearly a Doctor Who story as well. The Krikkitmen started as a stand-in for the Daleks, apparently, although I doubt they would have cared quite so much about the Ashes. Note the reuse of a Time Lord prison and its basis in slow time - here enough to cover an entire planet and preserve it past the end of the universe!