The best way to start writing a text adventure is to come up with a decent story and how the protagonist exists and interacts within that new world you have created. Then develop the engine around that.
I found that writing an engine from the ground up before I knew what type of adventure it would run led to me engineering in limitations about what the player could do. I knew how the story should play out, but had written a generic parser and system which was similar to other really basic text adventure games.
You can't beat a blank pad of paper and pen to hammer out a design first.