Nicholas Hoult and ATJ are only ever good in off-kilter roles like Mad Max, Nocturnal Animals or The Favourite. They're terrible straight men, and have been incredibly boring in superhero ensembles already, can't imagine them being great when they're holding the whole film on their shoulders. Also, wasn't there a rumour that Hoult hated working on X-Men? Who's to say he won't hate this too?
Sing Street's one of my favourite films of the decade, but Jack Reynor - as good as he was there - would be an awful Batman. Jack O'Connell could have been good, he's certainly a great actor. Probably the best from that short-list.