Eventually, Toffoli or Pearson, but not this year, and maybe not even next year, though they will have their shot...
Personally, I think Gagne is the best fit. They are familiar, and he has the skill to still play with them. His speed is complimentary to Carter's, and he has the flair to make plays.
Other than him, I think Clifford is your next best bet. His energy is hard to pass up.
Lewis has played well, but I don't put a lot of stock in his play thus far. He has had lucky bounces, not to take anything away from him, but we have seen far too much of the old Lewis to be convinced on him in a couple of games. A few more will give us a better idea.