A learning game? Do you have a target audience, because you can actually just cater it to them. As for learning, I'd probably go for something simple to program, like "Find 5 synonyms for this word", or even a "Find the meaning of this definition."
It doesn't have to be complicated in order to make people learn. It just has to show people something thy might not know. Usually if you pick a decent enough topic in history, you will be sure to get that learning in. Also, you get the choice on whether to make it interactive or text based.
A win-win situation?