I'm sure they will replace Marlin's sidekick with someone else - probably Nemo himself, who will be a bit older at this point and we'll see father/son interactions and the difficulties of being a single parent instead of father/outside the family interactions and the social adventures that come with that. The first film showed the obstacles Marlin had to overcome to break through the emotional barriers he built up since he lost his wife and his family and this next film could be about bringing him and his son closer. I feel like Disney has been thinking outside the norm of the tradition Disney families and characters and I'd like it if they could really hone in on the broken family aspect between Nemo and Marlin.

But this is all speculation. I loved Finding Nemo and I have hope this will be good.