Lets assume for a moment that there are negotiations between the Celtics and the Rockets (I don't think there are, but you never know).
First thing is that the Celtics will make Tatum unavailable for trade (which is almost impossible anyway due to his extension, poison pill). Then the Rockets will ask for the next best young piece, which is Brown. And I don't think there's anyway the Celtics could pursuade the Rockets to take Walker or Smart as center piece of a deal, neither do I think that a 3rd team will bring that young potential star on the table in exchange for Walker.
For me it's a given that Brown has to be in the deal from the Rockets perspective. The question is what the 'filler' is going to be. Just Brown + Theis + 2/3 guys (Langford, Nesmith, G.Williams, Pritchard, R.Williams, Ojeleye, Edwards, Green) would be the most straightforward way to make a deal.
Another way is adding either Walker or Smart to the deal. The interesting aspect of this, is that Walker is the better player but less valuable as a trade offer (contract, age, injury concerns). But including Walker can make it attractive for the Rockets if the Celtics take on Gordon's bad contract (59/3 left).
If it came down to either keeping/getting Walker or Smart + Gordon, what do you prefer?