A lot of X3 could be ret-conned with an X4 and not to mention we could get a new villain. Loved Magneto in the first 2 movies but hated how he was written in X3 so its time to bring someone else in now IMO. Sinister and especially Apocalypse would be great.
I think you've got to be careful with all this retconning.
I'm inclined to think Jean and Cyclops should stay dead. Cyclops will hopefully get to shine in First Class (and any subseqent sequels, they're probably aiming for a trilogy).
I also think Xavier should stay out of the picture in an X4, maybe appearing in an X5.
Magneto should be omitted from X4 or, if he has to be there, he could be a teacher at the school. Both he and Storm hated the idea of the cure, it could produce an interesting dynamic, as they dealt with it very differently.
I'd like to have seen Emma Frost, but as she was rescued by Xavier in Wolverine, she's likely to be in First Class.
X4 Villains: Sinister or Apocalypse or Hellfire Club
For X5: Shadow King or Proteus...(allows Xavier to return to the fray)
Singer would be fine, but would prefer adding a good executive producer more experienced at action and at making maximum use of the budget without frittering it away. Singer's budgetary control appears a little lax at times but just look what exec producing by Jackson got out of the District 9 budget and what exec producing by Spielberg got out of the Transformers budget.