I think it would be really awesome to see some more creative things like the choose your own adventure games, although unless you're gonna use voice recognition, it might be kinda lame to have to use dtmf for that. I would love to see an Adventure-like game that you could play by calling the pbx. Also, the search for the goog 411 listings would be really awesome. You should also have a meatphone function.
For the choose your own adventure it would be pretty easy to take something as simple as the popular line of books and do that - as those rarely had more than three choices if even that. The hard part would be finding something that sounds good for the text to speech.
Google 411: I have an AGI script for it laying around somewhere, it even cached results
Though it was some time ago, I'd probably want to rewrite it before giving it out - I think it was only like 15 lines of code so it wouldn't take very long.
Doing an Adventure-like game would be pretty tricky. Same could be said about TradeWars2002, though I think that would be easier to do than an Adventure game but equally hard to play over the phone. Given that you're going to have to listen to everything it would be best to stick to simple games - players won't be able to glance at the screen to get the relevant information or reread anything.
Though setting up a chess game would be pretty cool - its only 8x8 so it would be somewhat easy to set up an interface to it via the keypad, it would just be hard to remember where everything is..
Things like LoRD would be easy enough - similar games like Assassin - and there was a cyberpunk game - wouldn't require a whole lot either - though Usurper might have too many options to be easy to play. Barren Realms Elite, Falcon's Eye, SRE, etc would be possible, but might be somewhat hard to play, though that would be completely rad - particularly if we set it up to connect to other BBS's.
I think another cool feature would be a text file reader, and point it at textfiles.com or something.
Also some type of instant messaging that would alert the user if there was a message for them from time to time..And some type of voicemail (hehe we could do that encrypted voicemail nonsense I have laying around).
Yeah I dunno, if you wanna get working on this sometime RBCP drop me a line. I'd probably want to write it all as an AGI - most likely in Perl though I could be talked into C or if someone else was going to help develop it, Ruby. I must confess I've always wanted to set something like this up, but never really felt anyone would use it