Yeah, Theres really only two options for the future scenes to start off.  Post X3 or complete Alt timeline. Characters are  gonna drop like flies either way so probably wanna make sure the conclusion of the film has a possibility of Scott and other characters returning. Otherwise they are just making a larger problem.    Since we know The Wolverine is post X3 and ties into Dofp, I think that and the fact that they kind of need to fix some things for the franachise to continue will make them use a post X3 future. Its really the best way to go for the franchise...
		
		
	 
Going post X3 for the sentinel future raises questions that cannot be easily dismissed, or if they are then there's gonna be a fair bit of presumption taken that the audience is just gonna get how Xavier is still alive, how his new body looks exactly the same as before, how Moira went from being a CIA agent to a genetic scientist, and American to Scottish, without ageing much at all in over 40 years, Rogue and Magneto presumably getting their powers back, etc.
That being said quotes from Singer and Millar posted here do seem to support that it 
will be post X3 in 
some way. If that is the case that gives them the chance to effectively reboot the 'present' day franchise when the 1973 event is changed (perhaps the big event will be shown to be an important part of the original trilogy past)
The continuity errors can then be simply ignored, rather than needing to be fixed (not sure how they can 'fix' some of them in any event...Xavier walking at the start of X3 and still pals with Eric springs to mind). 
There is a 3rd option though: They could still go the alternate route (easiest one for explanation purposes) and still reboot the timeline from 1973. I think a lot will depend on just what the event is that they are changing. It could end up being something that gets changed in a manner that negates both the Sentinel future 
and the original timeline (if Mystique were killed for example).
Not sure if Singer would really want that though. Would mean he'd be effectively 'erasing' his X1 and X2, as well as Ratner's X3.
Anyway, atm I still lean towards them going the alternate route for the Sentinel future as it seems the easiest option to me  (I stand to be corrected as we learn more of course). How this film concludes and what that will mean for a potential 'present' day X4 remains to be seen.