My preferences:
Story(spitballing here):
- something involving dragons. Perhaps you are a dragon, or you befriend one. Other creatures could work, but I like dragons

- futuristic story in a similar setting to Dead Space (nice ship with advanced equipment and weaponry)
- PlayBook OS3.0 is coming out, what do you do?!?
after that last one I think I'll stop writing down ideas...
Price:
I'd probably be willing to spend $4.99 if it's done well, but there are a few thing to comsider:
1. Is there any replay value? Can you go through the game multiple times picking slightly different options and getting different results? Or can you continue playing after the main storyline is done?
2. How long does it take to finish the main storyline? If it will only take around 30 minutes to finish the game and there isn't some sort of replay value, I don't think you should be charging more than $0.99, if that.
3. How well does the virtual keyboard work for this game? If you have to use an external device to play it properly then you should reflect that in the price.
One more thing to keep in mind: If you make more than one game, will you sell each of them as a different app? Or will you sell a single app which then downloads adventures seperately? Or both?
And one question just because I'm wondering: I've seen text-based games with text input such as "throw rock", some with such things as "you now have the rock, press 't' to throw, 'd' to drop, or 's' to store in your inventory", and some with multiple choice and you either press 1, 2, 3, 4 or press buttons for the different options. Which type of game is this?
Sorry this is so long!