I would suggest writing up a very short game first, maybe 10 rooms with a handfull of items, something to let you get the game engine underway. By the time you've done that, you should have an idea ready, but it's something you have to decide yourself - what are your interests, what setting can you 'populate' with your own knowledge.
For me, I always liked the Hobbit adventure game, some great characters and locations. I would suggest going for a fantasy theme and keeping it kinda light-hearted, maybe a bit like the Discworld novels - real characters in extraordinary situations. I think that all the best text adventures had a lot of humour in them. The Magnetic Scrolls series was full of humour, and Hitchikers Guide, Holy Grail, even Colossal Adventure. I suggest playing a lot of adventure games and writing a mini-adventure before even deciding what to base your game on.

Health, Ammo, and bacon and eggs!
