Well, the wider your offer, the more potential users, but also here we need to consider that the biggest part of the planet has special requirements.
Children like graphics, stories, and games with "levels". My two are those who tell me what they would like to see.
Let me take Tux as an example:
Like Tux collects fish and gets points it could also collect correct words and destroy the wrong ones. It can pick the right equation results out of the sea (fish having the results on their side or back). Finished all 10 equations you go to the next level.
You can have various bags and collect words having to put them in the right bag.
There are many, many possibilities.
One thing to consider is the mobile sector - I just found this link:
which leads to:
The funny thing is: I actually thought about Nokia to talk to for mobile phone applications for the educational sector. Now I don't have the time to read up, but: this is really something to consider. In many countries people don't have money for a computer, but they buy mobile phones. It is one of the reasons why all our i-iter installations like http://dyo.i-iter.org also have a .mobi version:
More about the i-iter network can be found here:
So whenever you write applications: keep the mobile phone sector in mind - on the majority of the market it is the future. This was also confirmed during the last conference where we presented our projects: http://agis09.org .
What IMHO is also relevant is that the langauge for the application's UI can be changed without needing to have the locale supported by the desktop.