I plan to make Buu and Princess Luna fight on the day of his arrival to the pony world.
I'm not going to spoil things for you guys but here's what I'm going to let you in on.

He plans to fight and absorb the strongest fighter in the land to beat Vegito who is also teleported and transformed with Buu by mistake (thanks to Twilight). Buu hears that Celestia is the strongest and heads off to fight her in Canterlot, fortunately she is away and Luna is left holding the fort. Buu crashes through the army and confronts Luna and she held her ground very well... and that is all I'm going to tell you for now...

Background still needs to be done for this...

BLOG UPDATE

Since tumblr.com is being very difficult with me (not letting me customize the blog and not letting me upload posts) I've decided to move the blog here. It will have a special folder in my gallery and people can just send me notes to ask any questions that they have in mind...