Not to sound rude but Akame is kinda plain compared to the other girls in the show. Unless they give her some more depth and personality or add new and interesting characters then i don't see how you can make a good sequel.
I like Mine but her relationship with Tatsumi was very poorly written in both the manga and anime. But other than the conclusion we get with Mine and Tatsumi the manga is much better.
I don't hate Mine as a character but her relationship with Tatsumi is one of the worst written romantic subplots ever in a mainstream anime/manga. They didn't even come to the conclusion that they are even attracted to each other on there own somebody just tells them that they're supposed to be in love. I was sad when they died but if it was the only way to put an end to that dreadful romantic subplot then thank god one of them was killed off before the series finale. I'm glad somebody got something good out of it.