I think that Thomas wants, very much, to write a definitive 2A decision. He knows that the opinion that Scalia will be remembered for is Heller. Beside that, everything else is trivial. And Thomas will cheerfully take on the task of writing the decision that builds on the foundations laid down in Heller, because it will be studied for the next century.
This. Very much this.
Additionally, J. Thomas has been a staunch 2A guy for decades, longer than anybody else currently on the Court. As a matter of simple comity it would be a pure shame if the writing of the NYSR&PC decision were denied him.
In any event, the decision will have to respect the opinions (reciprocal comity) of the other majority signers, so even if J. Thomas is indeed given the writing, the result will certainly be less than he and, probably, most of us would wish.