I'd keep Mugabi too, as well as O'Donnell but strictly as a backup and an experienced head in the squad. I don't exactly know what Nicholson does yet but he comes across as a genuinely nice chap, a good talker and technically more than good enough so I'd like him to stay if he wants to.
Really not sure about Slattery, he came through pre-season lean and playing really well but soon slipped back into his sluggish routine of looking for free kicks. He'll possibly be offered a short term deal so that he can rehabilitate.