Plenty of people will say that they want to write a book without being able to tell you what it would be about, thinking that maybe they could just fire off stream-of-consciousness and eventually something might hopefully take form. IF doesn’t really permit that, but fortunately most people arrive at this goal with more in mind than “Hello, world! You are in: a place. There is: some stuff here. There also might be: an exit to somewhere else.”

As long as you can establish even just a setting, you may find many of the other necessary elements falling into place, the milieu suggesting characters and modes of interaction with them, puzzles to suit appropriate (or glaringly nonbelonging) potential inventory items or room descriptions, and hopefully… some sort of story to bring it all together. (Maybe even a title!)

Of course, game design seems like the most key element when you don’t have one yet; rather than making implementation the next step, it may be worth your while to revise the design and weed out great ideas whose workings aren’t going to be worth your time figuring out. (“I’ve got this great recursion problem where the player ends up in a room that’s actually /in his own pocket/ with giant-sized versions of his own inventory items! But what happens when he looks into his own inventory and tries to remove himself?”)