Bleach does a lot of things right, from some epic fights to truly memorable characters to one of the better villains in a Shounen with Aizen. What it doesn't do amazingly well is relationships. Few of them are canon and one of the ones that are isn't all that great (Ichigo and Orihime).
Debate on who Ichigo should have ended up with is strong among fans and one that we will touch on in this list. It's far from the only romance that was hinted at in the anime/manga but never truly confirmed. Some of these would have been great to see, others just leave you scratching your head.
10 Makes Sense: Momo and Hitsugaya
There's no hiding the feelings these two feel for each other, even if Hitsugaya sure tries to. In almost every single fight where the two of them are present, Hitsugaya is internally worrying about her, wanting to keep her safe. Much of that stems from being friends since they were young, Momo acting as a big sister to him in their youth.
During the Fake Kankura arc, Hitsugaya's energy spiked upon seeing Momo, a fact that Halibel commented on, but he vehemently denied. His constant need to deny his feelings leads you to believe there was far more than just a sibling style relationship between the two. This is especially true when Aizen manipulates Hitsugaya into stabbing Momo, look in his eyes was of pure horror.
9 Awful: Karin and Hitsugaya
This one is wrong on several levels, chief among them being her age. Through most of the series, she's between the ages of 11 and 13. That's way too young to be shipping with anyone that isn't in her direct age range. While Hitsugaya looks to be about 15, he's actually 100+ in Shinigami years. Two being a couple just feels gross on that front.
It didn't stop there from being moments in the anime where they hung out for entire episodes (mostly fillers). One, in particular, had Ichigo's other sister asking if they were dating, a fact they both denied. Thankfully Hitsugaya never seemed to return the feelings beyond friendship and Karin likely only had a crush on the younger Shinigami.
8 Makes Sense: Gin and Matsumoto
They are one of the more tragic potential romances in the series. For much of the series, their relationship seems rather one way, Gin being portrayed as the villainous snake who has no true feelings. You can tell something was there though from how Masumoto reacted to his betrayal.
That something is revealed to be undying love. Everything Gin had done the entire series was for Masumoto, only siding with Aizen so he could make him pay for hurting Masumoto when they were younger. Sadly, he doesn't succeed in his goal but does manage to come to terms with everything he did.
7 Awful: Shunsui and Nanao
This is another one that's just gross the more you think about it. While it isn't due to age here, though Shunsui is significantly older than Nanao is. It's that he's her uncle. There's nothing good about an incest relationship and it's rather surprising to see it be so prevalently displayed in the manga.
There's moments later in the manga where she states that she adores Shunsui and the entire fight the two of them have against Lille Barro is very based around the love they have for one another. It's clearly a relationship that Kubo wanted, but it doesn't make it any less creepy.
6 Makes Sense: Urahara and Yoruichi
This one is a little more subtle than any other on the list as there's nothing that outright points to their feelings as it was with Gin/Matsumoto or Momo/Hitsugaya. Instead, you have to infer it through title nuggets spread throughout the series.
What we do know is they are childhood friends (just like half the cast it seems) and there's a clear attraction physically, at least from Urahara. There are subtle things though, like how she doesn't go out of her way to tease him (unlike an Ichigo) and a scene where she seems to be wearing his clothes, something that's a common occurrence with couples.
5 Awful: Nel and Grimmjow
Via: Levilel
Frankly, this is a ship that came out of left field and while not gross like the first two, makes absolutely no sense whatsoever. Thankfully, this is more fan-made than something that Kubo himself made any hint to in the manga. It just seems like something thrown together since they are two of the only three Arrancar that survived (Halibel being the other).
Beyond that, there isn't anything here as they hardly interacted other than Nel breaking up a fight between Ichigo and Grimmjow during the final arc. Even then, she just seemed more annoyed with Grimmjow than anything. Fans will latch onto anything for fanfiction though.
4 Makes Sense: Ichigo and Rukia
This is the ship that sparks a great deal of debate among fans. On one hand, you have the few that liked that Ichigo ended up with Orihime, stating how long they'd been friends and that Orihime always had feelings for him. That's all and well, but chemistry needs to count for something.
Rukia and Ichigo had that in spades, showing both immense care for one another, but also goofing off with each other pretty regularly. Their interaction in the anime episode "Thank You" is one of the saddest in the whole series for a reason. In it, Rukia is saying goodbye to Ichigo as he slowly loses his Shinigami powers and thus his ability to see her.
3 Awful: Renji and Rukia
Many people harp on the Ichigo and Orhime relationship being awful, but at least its clearly indicated that they will end up together by the end, as unfitting as they are together. Renji and Rukia though? There's a true sense of love from Renji, especially during the soul society arc where he's fighting her brother, Byakuya.
After that though? It feels like the two rarely interact nor is there any real indication that Rukia thought of him as more than a friend through the series. Then all of a sudden they are married with a kid in the final chapter of the manga. While you can see how the two end up together due to their long-standing friendship, it's awful in its execution. Failure to set it up made their marriage feel like it came out of nowhere.
2 Makes Sense: Ishida and Orihime
As with Urahara and Yoruichi this one of the more subtle potential relationships in the series (one that obviously can't happen). Ishida seems to be one of the only people who truly understood why Orihime feels she has to help fight despite not having any fighting prowess.
He also seems to care about her more than Ichigo did, checking on her out of care rather than obligation. You can tell he liked her as well from how he reacted around her, often being flustered or blushing in her presence. Given his character, I suspect he never tried to act on these feelings out of kinship to Ichigo, who he knew Orihime liked.
1 Awful: Orihime and Ulquiorra
This is another of those odd ones made up by the fans. Orihime and Ulquiorra had an interesting dynamic with each other, both affecting the other in some way. Ulquiorra helped show Orihime that not every one of their enemies was some heartless or bloodthirsty creature, they were people just as she was.
She helped show Ulquiorra what a heart was, something he'd desired for his entire existence. Some may interpret that as him having feelings for her, but that was never shown, nor would it fit his character. He was very much a Frankenstein style character, a monster trying to understand what he was.