I like the idea of taking a proven computer flash game, that you know is good and putting it on the psp. I think it is the best kind of homebrew. Instead of taking a stab in the dark about gameplay I think "The Last Stand" would be a great game for the psp
http://www.addictinggames.com/thelaststand2.html
I can easily understand this game would be pretty hard to do esp. because of the control difference and graphics. I think you could cut out some and still keep the gameplay. It would be a pretty sweet game if accomplished. Just an idea though