Singer's basically flat-out said that he's going to maintain at least SOME of the existing continuity, so completely tossing everything that happened from 1973 on - or tossing out just the events of O:W or TLS - isn't likely to happen.
What I do think is likely to happen is that he resurrects certain characters and offers an explanation for certain things that fans continue to complain about (Xavier walking), and makes some changes to the events of TLS so that they happened, but they happened in a slightly different way to what we saw.
Someone posted the following in a thread I started in the DoFP forums, and it's something that I not only think could very possibly happen, it also very much fits into what Singer's said about what he wants to do insofar as the timetravel and 'fixing some things' is concerned:
2023: Failed Future. Wolverine fails. Everyone dies. Bishop and/or Blink somehow relays the information from this future to the past at some unknown time frame prior to TLS.
OT timeframe (implied but not seen in the trilogy): The information from Bishop/Blink is intercepted by Xavier, who begins Danger Room sequence training to prepare mutants for the Sentinels.
1973: Wolverine tries to stop Trask along with the rest of the X-Men, but it leads to Stryker discovering the power of Cerebro and going through with X-2. Sentinels are postponed but not entirely put of the picture.
2013: Wolverine is sent back to 1973 from the original timeline by Kitty and another mutant (possibly Xavier's twin). Xavier is already dead in this timeline. This is the twist of the movie. The time travel had already taken place.
DoFP (present day): We are in the altered timeline. Wolverine was successful preventing the war from escalating into the events of the OT. Magneto/Xavier end up working together in this new timeline after the events of X-2. Cyclops is alive as is Xavier. However, work remains to eliminate the Sentinels entirely. The plot also revolves around moving Wolverine back in time to complete the "time loop" to ensure he completes what is intended in the 1970's. However, in the new timeline, Kitty is injured, and they need Rogue to complete the loop.
In summary, this allows you to cut between both the past and present scenes, while maintaining the suspension of belief. The future is now unraveling before our eyes while we cut to what had taken place in the 1970's. So there will not be a separate "alternate" timeline. The timeline we see in the present will essentially be the "new" timeline where the changes have already happened, but more work remains. The original timeline will be a flashback, last minute reveal that bridges all the dead characters that have returned in the new timeline. That's what I predict...