Technically, if Sony let the trademark go, it's up for sale/use.
The game itself is still under development apparently, so if it ever sees the light of day, it may be under a different name.
If MS wanted to waste money buying the trademark for no reason other than pettiness, they could, just to use it for Halo 5.
Of course making a deliberate attempt to call attention to a game most have given up on playing would be a little too subtle for the average consumer to make it worthwhile.
Plus, there's always the odd chance that a renamed "Last Guardian" shows up at E3, and an even smaller chance said appearance comes with a release date, which would make for even greater comedy when that blows up in Microsoft's face.
I think MS is better off focusing on what they're releasing rather than wasting time focusing on what the competition may not be releasing.







