Probably will never happen but:

Btooom! is a really good anime but it's incomplete. I adore the manga. It's a fast paced action packed type of manga. The cliffhangers have me on the edge of my seat everytime. And I'm like "NOOooo I need more!" It's one of my favorites. I read it online and recently started buying the manga series.
The anime only covers a really small portion of the beginning of the manga and just falls off the face of the earth.


This one doesn't need a sequel but I would love another season of it:
Shiki <3 This bloodbath of an anime is one of my favs. I adore it.

Haganai needs another season.

Too many, but the first to come to mind is Baccano! Adapted from novels written by Durarara!!'s author, but is less received than DRRR, though it's just as good!

Most of the animes I can think of are ones that have second seasons but haven't been dubbed in English since the anime was not popular, etc that I would love to see a second season brought over in English:

Saint Seiya

but there is a few animes, can't remember the exact names, that need second seasons cause I feel the story just ends and leaves questions that need answered. As soon as I get the names, I will update this.

Yuuri on Ice

It made me happy for how short it was.

Ouran High School Host Club

It's probably already getting one but Cardcaptor Sakura Clear Card Arc needs a third season.