For the longest time I really wanted a Bleach x D.Gray-man crossover.
More specifically, I think Allen would be an unusual character in the Bleachverse because of his strong belief to save everyone. No one really questions Ichigo's moral compass much, but I think with Allen being there, maybe if they start fighting side by side, they could learn something about each other.
For a more light-hearted crossover, I think Majin Tantei Nogami Neuro x Beelzebub is a perfect combo. You have sadistic demons who come to Earth for some kind of lifestyle reason, and the humans who suffer having to deal with them. Plus, I think it would be kind of hilarious to see how Neuro acts around royalty. Maybe we'd find out that the demons of Beelzebub don't eat food either, but other kinds of emotions and stuff
We all know Alaindelon feeds off of lust, don't we.
Vinland Saga and Berserk, just because it would be pure win
Aside from that, it would be interesting to see how Thorfinn and Guts interact
Berserk and Claymore can be epic. Both has similar setting, though Berserk is obviously so much darker than the latter.
Bleach x FMA
toshiro and edward the two shorties would have a blast of a time with each other and alchemy vs shinigami would be funny especially since so many times ed says i dont belive in an afterworld or whatnot haha he'd have a heart attack
One Piece x Naruto: Could be both good and really funny.
Bakuman x Death Note: Mashiro and Takagi drawing Death Note could be cool?
Prince of Tennis x Hajime No Ippo: Who has the toughest work-out? Also it could be REALLY funny.
When I was thinking about these three fantasy crossovers, I actually started to really want them to be done!
Bleach x One Piece -Aizen trolling the World Government would be quite a sight. With Teach there as well, just think of the possibilities.
Shin Angyo Onishi x Berserk x Samurai Deeper Kyo- All three combine to form one giant crapsack world. I'd totally pay to see a battle of villainy between Aji Tae, Former Crimson King and Griffith.
I kinda want to see a crossover between GE ~Good Ending~ and SKET DANCE. The core reason is because both series take place at a school in Tokyo, and the protagonists are the same age. (16 - 17 years old.) Humor takes precedent over romance in Sket, but there's still some there. What's interesting is that GE's characters tend to see love from a "mature" perspective- which makes sense considering the demographic of Weekly Shonen is a tad bit older than JUMP.
I don't think the characters will be able to mesh well together so a cameo or a mention of the other school would be interesting.
D. Gray-man and Ao No Exorcist. They're from the same magazine so if Jump SQ ever decides to have a collaboration thing like One Piece/Toriko and Gintama/Sket Dance, it could very well be possible.
The exorcists could do exorcist stuff with other exorcists; the Millennium Earl could chill out with Mephisto;
Allen and Rin could angst with each about killing their father; all in all, it would be very awesome.