Well, if we're going to have those types of magical-girl entries, how about Kaitou Saint Tail? (Just be glad I'm not suggesting Tonde Buurin... Or maybe Moetan.) There's also the Mermaid Melody series, though G-Force 1 already attempted it, somewhat, on fanfiction.net.
Shuffle?
(Come to think of it, did anyone actually attempt a Ranma/Shugo Chara crossover?)
And, if we're including Macross 7, how about Macross Frontier?
Hmmm... how about...
Transformers: Masterforce (if I'm remembering the series name correct... the one with Minerva and the other kids who become TF heads.)
Majin Tantei Nogami Neuro
Idolmaster Xenoglossia (I wrote a bit of an idea on this one, myself...)
Getsumen to Heiki Miina
Telepathy Shoujo Ran
Jigoku Sensei Nuube
Asura Cryin (probably misspelled that one)
Akikan