Manga wise:
Does wanting a story to pick up again count? Because D. Gray Man needs to be serialized again and HxH needs to be finished.
But as for sequel, I think Katekyo Hitman Reborn is in dire need of one. The ending was just so anticlimatic and horrible that we're left with the feeling of emptiness. What even happens to Tsuna's story anymore? His entire development was thrown out the window.
I don't want for Bleach to have a sequel- seeing Ichigo mature, grow up and have kids would be nice but I would hate for a Bleach sequel. It would suffer the same fate as Naruto Gaiden- everyone's already OP so who would they be fighting anymore? It would feel like an endless cycle.
Anime wise:
I want a sequel to the original Yu Gi Oh. Like what even happened to Yuugi after all these years? Does he have a family? How long did he stay a duel master??
Yu Yu Hakusho needs a sequel. I'd even take a reboot. But let's be real here- I'd rather see HxH be continued and finished.