^FLCL was only meant to be a 6 episode OVA and it ended perfectly there.Maybe some fan rumors about a "sequel" may have reached your ears,but it's not true.It is after all one of the manga/anime/ova/games that has the "Dragonball ending"*.
I still wonder,why people are ****ed about the Death Note anime...it is the only media that Kira is alive after all...
As for the Shaman King manga,it also suffers from the premature manga syndrome...but with a reason.Up until last year,Shaman King's popularity had hit rock-bottom...until a miracle[no,seriously...it IS a miracle] and it was revitalised and had a proper story,uncencored and with the proper ending and dialogue.If an anime is made now...that is unknown for the time being.
As for D.Gray-Man and Soul Eater...plagiarism issues in the manga were forced to end the anime...ironically they were suppossed to be plagiarised scenes from the Trigun Maximum manga...anyway...at least the manga continue.
Oh yeah...and Gungrave.It ended too soon and totally on the wrong side.Where's my Gungrave : Overdose anime????Where are my Gungrave games?????
*Dragonball ending : Although the project ends and reaches it's conclusion,the words and images before it's closure say : that's the end of what I can say...many things happen after that,but it's up to you to imagine them...
And it let's the readers fantasies run wild,reaching a far better conclusion that anything else in this world.It truly becomes and endless adventure.