I kinda see a fishmen joining Luffy's crew very likely. I mean, WB was the one who stopped the racisim and mass massacre on Fishermen Island. Now that WB's gone, as we've seen, his declaration means nothing. Although Jimbei went back to defend it, his words as a former shichibukai won't even have half the effect of WB's. I see the strawhats protecting the island, then maybe declaring their protection on it, and leaving Jimbei behind to defend it, but Jimbei will have his few found first mate join Luffy's crew. It's logical, makes sense, but Oda is unpredictable.
I think a Dragon DF is unlikely, simply because everyone is expecting, and Oda won't do it because of that. I'm not so sure about having a logia join the crew either, especially since Oda hasn't proven that Zoro or Sanji can deal with logia's. Add to the fact that Nami won't be able to hit them on the heads when their doing their comedy gags, it just wouldn't work.