In terms of shows opening later this year, might be worth checking out the all-female Company, Jamie Lloyd's series of Harold Pinter short plays, Don Quixote, The Height of the Storm (all West End), Hadestown at the National, and the brilliant Summer and Smoke and The Inheritance both transfer to the West End. There's also the RSC's Romeo and Juliet at the Barbican, and the wonderful War Horse returns if you haven't seen that (National).
Long-runners wise, there's Everybody's Talking About Jamie, Dreamgirls, 42nd Street, Tina the Musical, Bat Out of Hell, Les Mis etc. and it's easier to get Harry Potter tickets now that it's on Broadway.