The only thing that I can think of that they could ever remotely possibly do with a plot is if Nemo is grown-up, has offsprings of his own, and one of them gets lost and they have to search him it... that's all I can think of... I mean, Nemo is one of my all-time favorite Pixar movies (I LOVE aquatic tales), but I think it falls into the category of movies that really shouldn't have sequels... like Ice Age... and I hope to God Roger Rabbit (I still hear rumors Zemeckis is going to do a CGI sequel, but I never hear anymore advancement on the rumors or anything).
I think Roger Rabbit does deserve a continuation, but ONLY if we can have the same circumstances that made the first one. Co-operation between various licensing companies for one. If we can have Sega, Namco, Nintendo, and Capcom characters (to name a few... Sony owns Q*bert) in Wreck it Ralph, we should be able to get WB, Disney, Classic Media, and whoever else is out there together. Secondly, they NEED to make Roger and the Toons traditionally animated, or it will lose all the point of being a Roger Rabbit film. And thirdly, the ideal RR sequel should be about Roger and other theatrical toons coming to terms with the advent of television. Along with cameos by HB characters and Bullwinkle, of course... they're major TV players around the time.
As for Nemo... it doesn't need a sequel at all, but I found this out looking it up...
In 2005, after disagreements between Disney's Michael Eisner and Pixar's Steve Jobs over the distribution of Pixar's films, Disney announced that they would be creating a new animation studio, Circle 7 Animation, to make sequels to the seven Disney-owned Pixar films (which consisted of the films released between 1995 and 2006).The studio had put Toy Story 3 and Monsters, Inc. 2 in development, and had also hired screenwriter Laurie Craig to write a draft for Finding Nemo 2. On July 17, 2012, Finding Nemo 2 was confirmed. Andrew Stanton will return to direct the sequel.
YEP! Same circumstances that almost gave us the BAD Toy Story 3. So, it was another "Did someone say CHALLENGE?!?!" situation.
Still... the movie that demands a sequel is Incredibles. There's a lot of ground there.