I think an interesting choice would be Murat Yakin- we were linked with him pre-Koeman but he joined Spartak Moscow- didn't do so great there but he did an excellent job at Basel before that when they had a great run in Europe and played some really good football with Salah, Elneny, Schar etc. Although he's currently managing in the Swiss second division with Schauffhausen I'm pretty sure he went there to help out his brother (Hakan) with coaching, after he joined I think they went from bottom to 4th. Probably too much information but I think he'd be a good fit.