If you've already made one, I'd be interested in chatting with you. I may have an offer you like.
When you talk about the 'text-based games where you rob a bank, etc', it sounds like you are referring to Torn, the browser game that was posted on HN a few days ago for making its creator a millionaire. There's quite a fundamental difference IMO between clicking buttons on a webpage and entering pseudo-english sentences at a CLI.
I feel I can speak with a bit more authority on the first kind as over the last few months I've been working on my own browser game, which currently has around 600 active users (active = logged on in the last 5 days). It's not purely text-based, as the main screen includes a 2d map showing the area around a player. Still, a few people have said it reminds them of the old Infocom games, which I think is pretty cool.
If you want to take a look, the game is called Shintolin (http://www.shintolin.co.uk) and is set in the stone age; players craft spears, build huts, hunt animals and so on. One of the features is that every item and building in the game is player constructed - there are no NPCs apart from animals, and the game's history arises naturally from the interactions between the different villages.
Sadly it hasn't made me a millionaire, but I have learned a lot about programming from it, especially how to organise large projects (I had to scrap the original version of the site, after it became an unmaintainable mess of PHP scripts). There's no Javascript or Flash involved, just plain HTML/CSS, with a Ruby and MySQL backend.
Anyway, I mention that just to show you that the space of possible text-based games is quite large and diverse, so you might want to look around to see what tickles your fancy. Focusing purely on the gameplay and content without worrying about graphics gives you a lot of freedom (though HTML makes it fairly trivial to create a basic GUI).
One thing I've been getting into recently is roguelikes such as NetHack, which do have graphics, but very minimal ones (usually entities are represented by ASCII characters). As they often feature procedurally generated content and AI, I think they could be an interesting project to work on from a CS perspective.
Also, apart from a few recent games, not many text-based games have been well received, and they likely never will be, so I suggest you try doing some casual games in flash. You could look at lostgarden.com for some inspiration (and also monetizing tips).
It could be quite interesting to see what is possible with modern languages and text-based applications.