Rukia Kuchiki may not have become the protagonist of Bleach, as originally planned, but she sure makes her mark on the narrative in several ways. She's strong, versatile, compassionate, but most of all, she understands that friendship and loyalty are superior to following the meaningless traditions of Soul Society.
Several fans wanted IchiRuki to become a thing, but this did not happen — to nobody's surprise — because Tite Kubo had been foreshadowing Rukia's inevitable love affair with Abarai Renji from the very beginning of the story. Still, this relationship works because they had been together for several years before becoming Shinigami. What would her alternative romantic partners be like, though?
10 Orihime & Rukia are equally sensitive & kind, making this union more likely than the ones they are given
Inoue Orihime and Rukia are an excellent match, mainly because the men they are paired with have the annoying habit of running off to fight without warning, then getting themselves hurt for no reason. Nobody needs all that unwanted stress.
Besides, it's not like these two are cowards or anything; it's just that they possess common sense and tactical planning skills. On the emotional front, both women are equally sensitive and kind, making this union more likely than the ones they are given.
9 Rukia will be able to understand Ganju's perspective without much difficulty
Like Renji, Ganju is quick to anger, is overly proud of his abilities, but paradoxically despises the Shinigami of Soul Society. He might refuse to partake in the idea of being romantically involved with someone like Rukia, but then again, she's not born and brought up in Sereitei.
Basically, she understands the suffering of those who grew up in the crowded zones of Rukongai, and will be able to understand Ganju's perspective without much difficulty.
8 Rukia is certain to appreciate Kira's peace-loving mindset
Kuchiki Rukia can be hot-headed, although rarely, which is why she is in conflict with her partner quite often (as well as with many of her friends). In comparison, Kira happens to be a quiet and introverted character, and on the surface seems like he has no ambition, motivation, or interest in his job, whatsoever.
Regardless, he understands the importance of fraternal loyalty and utterly hates the fact that anyone has to fight, ever. Rukia is certain to appreciate this peace-loving mindset, even if it's shrouded in indecision.
7 Uryu has proven himself to be a gentle soul
Why shouldn't Uyru be set up with Rukia? Irrespective of their disparate apparently incompatible histories, being Shinigami and Quincy, he has proven himself to be a gentle soul, whatever his father or Ichigo might believe.
He is carefully measured with his planning, and, as a consequence, will not make his partner any more anxious about their fate than they have to be. There is also the shared bond of trauma between them, pertaining to their experiences in the Soul Society and Hueco Mundo Arcs.
6 Shinji is as determined & discerning as he is light-hearted & sarcastic
Shinji is technically too old for Rukia, but is that even a thing in a world where people live hundreds of years without aging in the slightest?
The great thing about him is that he's as determined and discerning as he is light-hearted and sarcastic, which shows that his personality is rounded better than most other characters (who are usually monotonous: angry Yamamoto, haughty Byakuya, and so on). Rukia's interest will not wane since Shinji will keep the experience lively, at least as long as he doesn't annoy her too much.
5 Rukia has the emotional depth required to take care of Hinamori
Ignoring Hinamori's supremely weird fascination with Aizen, it would be prudent to pair her with Rukia, simply because the latter has the emotional depth required to take care of her.
This is obvious in the manner in which she goes into a flurry of fury for no reason, in part due to the fiery nature of her zanpakuto, Tobiume. In this respect, Rukia's ice-based Sode no Shirayuki grants her an eerie level of calmness (usually, but not always), solidifying their relationship by providing contrast and balance.
4 as perseverant as he is, Chad knows when he is beaten
For all his muscular build and overwhelming physical strength, Chad is a placid individual, so much so that he regularly allows himself to be teased or insulted, preferring not to respond in kind. This coupling is ideal for one main reason; as perseverant as he is, he knows when he is beaten.
Like Rukia, Chad adores anything "kawaii," such as cute creatures and stuffed toys. All they need to establish an eternal rapport is to fill their living space with a horde of bunny plushies.
3 the crucial factor is how agreeable Nelliel & Rukia are
Contrary to her child state, Nelliel is sensible, smart, mellow, rational, and unflappable, regardless of the situation she finds herself in.
Most important, however, is her enduring philosophy of nonviolence, which is a trait that Rukia certainly needs in her life, given how much she has gone through in the span of two years. Sure, they'd also make a rather cute couple, but the crucial factor is how agreeable they are (which is a lot, surprisingly.).
2 Bazz-B exhibits an inordinate amount of care for a person in his situation
Bazz-B is basically another Renji, from the flashy hairstyle to the overconfident expressions of masculinity. So why would Rukia choose someone exactly like her current partner?
Well, mainly because he exhibits an inordinate amount of care for a person in his situation, such as when he protects Haschwalth, or when he willingly shifts loyalty away from Yhwach on account of his comrades and himself being betrayed. To be fair, though, he has a lot of personal work to do before being a feasible match for Rukia.
1 Matsumoto & Rukia would look adorable together
Matsumoto is playful, buoyant, and amicable, and she and Rukia already get along great. Although the Squad 10 Vice-Captain is more frivolous about her career, it makes no difference to the way she manages to keep her team organized and well-oiled.
Now that Rukia's the Captain of the 13th Squad, and has no subordinate of her own, it would be fantastic if Matsumoto transferred out from under Hitsugaya, who would honestly prefer someone with his work ethic, honestly. Not to mention how adorable this pair would look together.