It was hinted in a recent Podcast Unlocked that we'd be very happy at what's coming up for GWG in June. Makes sense, means the undecided people that watch the E3 conference will see '[good game] is free on Xbox, look at how awesome GWG is! Maybe they'll give me more great games for free - better get an Xbox!'.
My bet is Tomb Raider. A very good game, looks great, would give people time to finish it before Rise of the Tomb Raider and probably boost pre-orders for it. No DLC to sell, but sales for that game must be bumping along the ground at this point anyway.
Moonshot? Sunset Overdrive. Would look great for E3, and it has a fair amount of DLC that can be sold. Seems to go on sale fairly frequently at this point anyway, it's third party exclusive, and pretty much anyone who wanted it (who owns the console) has already bought it. Would push a lot of people to buying Xbox One's, especially those in the Sony crowd since it's an Insomniac title. Conspiracy theory, but it would also explain why we're getting a 'limited edition' Halo MCC white Xbox now - the Insomniac ones that didn't sell could have just been repackaged, new download card put in, new box.
Either way, I hope we go back to getting 2 new games a month. Maybe that'll be the 'big surprise' - I wouldn't mind it.