Really, if Microsoft still have a large amount of stock left by the time they release the slim, they always have that landfill...
but really, they could insta price drop the current bundles by $100, and since they wouldn't be restocking those, they would immediately get replaced by the slim.
I think the slim will be $299, but I think for it to have this "Oh shit, this looks attractive" look to consumers even more so, they should bundle Halo 5. I'd bet that MS will have a segment this E3 for Halo 5's continued post launch support, and with all the new additions they'll probably show, an announcement that it'll be bundled would be icing on the cake. Almost every new Xbox One owner would be guaranteed to have Halo 5, and they can make money off of the REQ system. More Halo owners can then help secure even more potential Gears of War buyers this holiday, and would be Microsoft's answer to Sony now having Black Ops 3 bundled with the PS4.