I like the idea of shifting alliances and pushing for an aggressive start. I am thinking during the 7th year would be the best place to start. Here are my ideas based upon the list of suggestions you gave me.

1. I can seeing him kidnapping Luna when she first comes back to Hogwarts for her 7th year. Maybe on the train? 

2. I like this idea as well. Maybe using this motivation as a reason for kidnapping Luna. He is using her to prove his value to the Dark Lord? Showing he can convert people? Just an idea

3. I liked the idea of killing Ron off until you mentioned him going to serve the Dark side, and I like that idea better.

4. With this tension between her and Harry, would it make her want to leave him? How would be keep them together if she resents him?

5. I think the 7th year is the perfect place to start, maybe right around the time Ron leave them? Or maybe a little before so we can get our roleplay going first. What do you think? 

6. I think this point is the most interesting. I could see him deciding to do this later on in the role play. Let's say after Luna and Ron have converted to the Dark Side? He decides that he wants to become the Dark Lord and goes to take out Voldamort. This is an idea I would really like to explore

7. I think I like the idea of Ron turning to the dark side a lot better then just killing him off. I say this because it could be fun to bring him back in periodically just to reemphasize the tension of betrayal. 

8. I am not sure how I am going to write Luna. I want to give her more substance then just this quirky girl who says random things that no one understands. However, placed in Draco's hands I could easily see her character developing into his evil queen. I could see her developing into a force to be reckoned with because she would do things most others would not. I could see her standing up to Draco rather then caving under his attacks.