That would make little sense imho. First of all it would require a tremendous amount of effort realigning all the three different core architectures.
I've heard several developers (both Maya and XSI devs) state that not only would this be nigh on impossible, it would be very, very cost prohibitive. The result would probably be quite the Frankenstein app under the hood. The last thing you'd want from a 3D app (one that will be expected to process and manipulate vastly complicated geometric assets) is a cobbled together core. I know I'd probably loathe to use something like that.
Right now AutoDesk can make the most money doing what they've always done. Sit on their products, add useless gimmick features like their Borg viewcube (I dread to see that appear in my XSI 2009 viewport) and charge their annual over the top subscription fees, knowing most of their clients are too dependent on their products to make a quick and clean run for the competition.
Eventually a next generation application will become a necessity as Max and Maya's already ancient cores grow too unwieldy to expand upon any further.
At that point AutoDesk could of course use all the acquired technology and patents to write a next generation application to supplant Max, Maya and XSI.
But I don't see this happen anytime soon, if at all. Redesigning and writing an application as complicated as these three are takes years to do.
Just look how long it took SoftImage to develop XSI to the point where it became just about usable in production and what it did to their market dominance (Maya slaughtered them).
What I think AutoDesk would do instead is doing what they do best. Which is using their anti competitive market dominance to pummel the competition's market share and their stock prices for a few years and snap up the remains for a bargain only to ride that horse out for years until it's made them enough money.
That's their modus operandi.
Unlikely seeing as about 40% of all Maya sales are for the Mac edition. I think they will just merge them all into one huge package and sell it on all platforms.