You could talk about different genres, such as yaoi, shonen, seinen, josei, shoujo, magic girl, school. You could also talk about many of the themes in manga and anime, like the power of friendship, the power of words, the power of love etc. You can talk about the differences between manga and anime (especially of the same series) and you should also mention notable works and what they're like as examples. You also may put how it's changed through time, including style, material and opinions. You can also compare it to manwha and manhua, which are Korean and Chinese versions of comics. Oh, and you can compare subject matter between Eastern and Western comics.
This site should help with ideas too:
Anime - Television Tropes & Idioms and
Manga - Television Tropes & Idioms.
Hope this helps! Feel free to ask if you need more ideas.