He is not coming. I know loyalty means nothing in football these days but he is one I believe on his word. He just signed a new contract and says he will stay.
Never going to happen, but I think Benitez would be the ideal candidate right now. His contract expires after this season and he is keen to come back as his family resides in Liverpool. He is also somebody who tightens up a team and improves them defensively which is Arsenal's main problem now.
Signing a new contract mostly doesn't mean that much, and especially when Dortmund is just a few points above relegation spots.
Currently Frank De Boer from Ajax FC is going to leave after this season, he might join Tottenham, but Arsenal would be a better choise.