Cowboy Bebop should NOT have a sequel or a prequel, it is fine as it is.

I think the obvious ones are Berserk since the animators did not follow the manga and left things hanging, and Hellsing.
The problem with Hellsing is that there was so much room for character development and yet it was cut short.

I also would like to see more Hajime no Ippo I am sure movies will come out in due time.
I have not seen the entire Kare Kano but I understand that it was cut short, hope they expand on it.

Outlaw star could use a nice sequel.

that's all for now.