I'm honestly in agreement with Ash not capturing the Phantump. As The 4th Kira has stated, Ash isn't interested in capturing every Pokemon he sees. I'd love to see him capture another shiny, but I highly doubt he will. Unless they're trying to keep the capture a secret this time around, I'm not sure if that's been done before, but I can dream?
Suppose we'll see the OP updates, since those always seem to spoil captures. One thing I do know is that Ash will eventually have a sixth Pokemon, since he'll definitely need one in time for the Pokemon League, for instance...
I do feel Ash still needs a Grass type Pokemon, since he hasn't had one this generation yet. :x He has three Flying type Pokemon this time around, I wouldn't want him to bring back his Charizard, since that'd up his Flying types to four. That's kind of why I'd love for him to get a shiny Phantump in the first place--it's been since, uh, Johto? Since Ash had a shiny. I wonder if Ash would evolve the Phantump, hmm... probably not.
I also don't think Goodra is going to come back. Ash released it. He never came back for any Pokemon he's released. Gliscor was different, since he sent him away in training, not releasing it. I feel like it is highly unlikely such event will occur, but I suppose anything is possible, you never know.
Also, in regards to Serena.