Although the idea of making cash by making a blog about something profitable sounds enticing, I honestly think (even though this sounds really corny) that you should just make a blog about something you like. After all, if you really like doing something, you will want to put more effort into doing it right. This leads to higher quality posts, and high quality posts can make a high quality blog.
But regardless, I'm a real sucker for philology and etymology, so why not write about some fun linguistic nuances in English and German or something. Like, for example, why do the words 'knot' and 'knave' have a weird ass silent 'k'? Maybe something about declension in German and how it affects sentences.
I dunno, nerdgasm over, but seriously, I'd strongly recommend writing about something you would have fun doing.