I'm in the "probably nothing camp" and its simply because i'm being realistic.  Sure you can think up hypothetical stuff but if it goes too far whats the point?  Is it useful to say "hey if they paid me to take one i would"?  I think for the thread to have any useful discussion the more the "what if..." Is possible the better.
		
		
	 
I agree that it's not very useful to list things that clearly would never happen. However, I think that if you are someone who still has some passion for playing games, it shouldn't be too hard to think up a realistic scenario of something Microsoft could do that would make you buy an XB1. Maybe your purchase is earnest becaue they have introduced something new and exciting. Maybe it's grudging because they have purchased a developer or game you were already excited about.
For me, I decided to buy a PS3 when I saw The Last Guardian for the first time. Obviously that turned out to be a bad factor to base the purchase on, but it's an example of a game that looked good enough at a conference to push me to purchase a system.
Recently we have seen leaks of Project Beast, which presumably will be announced at E3. Now, I already own a PS4, but if I didn't, I can imagine that a game like that might be enough to convince me to buy one, since I love the Souls series, but really want to see it reinvent itself.
As I've mentioned earlier in the thread, I bought a 360 after watching the E3 conference where they showed Braid, Bionic Commando, Castle Crashers, and Geometry Wars 2, and they were all coming out that summer (those were not all brand new reveals, but it was the showing at the conference that convinced me that they would be awesome games). Incidentally, this was also the same conference where they announced FFXIII would be coming to the 360 - also a bad motivator in retrospect, but still a big deal at the time.
I had no plans to buy an N64 until I saw a demo of Mario 64 running in a shop. It blew my mind, and a couple weeks later I was standing in line at launch.
Can you really argue that those sorts of things are unrealistic? They actually happened. So I wonder why it is so difficult to imagine a realistic situation in which something similar happens for the XB1.
I was surprised by a lot of stuff at last year's Microsoft E3: Sunset Overdrive, Quantum Break, Crimson Dragon - None of them were system sellers for me (yet), but they certainly showed me that Microsoft is capable of pulling out surprising games in a conference.  I mean we already have rumors of an exclusive game from Platinum, right? What if that game looks totally amazing? Doesn't matter to you? If Platinum is not your cup of tea, pick your favorite independent developer. Imagine this generation's Gears of War or Fez or Crackdown or EDF 2017 or Tales of Vesperia or whatever genre you are into. Still doesn't matter? Then what actually does?