I'm willing and ready to include all of my characters in the plot, I have a few suggestions, though ideas are welcome since I haven't really thought this through.
Mariana Diggory,
She is a nurse, I have no idea how to directly include her in the plot apart if I do it through you K. Even without the direct involvement in the plot, she can always tend to those who survive werewolf attacks or rather help anyone who tends to the victims (though from what you told me, those aren't so numerous). Mari is also Order member, but I am not sure how is OotP involved in the plot.
Gabrielle Delacour
We can include her in the plot more directly. She is a skilled fighter, former Auror and works at the ministry and I wouldn't mind involving her in a fight or in a rescue mission if you need one, or in sending Blood's followers to Azkaban. She can be equally effective in either of this activities.
Lilyana Volkova (once she is approved)
She is too young to enter any direct confrontation. She is (will be) a Dark Follower, and supporter of Death Eaters, but she is not Blood's follower. I plan to make her very influental among students, maybe even leader of DF if opportunity arises, but again I don't know how to directly or indirectly include her anywhere..