I Think It's Tsurara/Yuki-Onna.
Because I think the mangaka already gave some obvious hints about RikuTsura... and eventually in the end, rikuo will realize his feelings for her... and Come On! If Rikuo marries Kana or Yura, It's the end for the Nura Clan... And what's the point of breaking the curse,if he doesn't need it to be broken? Rikuo is not an idiot to marry a HUMAN and end their bloodline... He'll be just doing what Hagoromo-Gitsune wants him to... and besides, he might even have secret feelings fo Tsurara
If the third heir is married to a human and he is 1/4 human then their bloodline will become too human and unfit to rule the yokai. He defeated Hagoromo Gitsune, so didn't that end the curse? Also, the second heir was married to a demon but had to marry a human so he could have an heir.
unfortunately that scene with Kana and Rikuo never existed. he never even gave her a mirror. & he brought her to the yoaki bar to distance her from him.
kana is hardly important to the plot anymore. if he wear to bear child with kana, the bloodline of the nurarihyon will go down to a 12.5 for the offspring.
anyways, to the question, me being a rikuo/tsurara supporter, i say tsurara. yes she is a yokai, and i'm aware of the curse. but feel like mangaka has really shown too much ship tease to not make end them up together. hmm how? a wedding? why? because they have a lot of shippy moments. umbrella scene is one of my favorites. and when tsurara got hurt badly, he snaps. its shown that both support each other and have an amazing bond. <3
I pick Tsurara! Why you ask? Of course, if he marries Kana then their son will still have yokai powers but only 1/8 of his blood which will make him insufficient in becoming the next leader. If he marries that exorcist(I forgot her name..) it's just like fire and ice, IMPOSSIBLE LOVE!! If ever they'll have a son, he'll still be 1/8 yokai..=.=" and most of his blood will be from his momsy and the rest goes to his dad..which is quite small in percentage. Plus, they'll have family feuds to choose whether their son will be the heir of the exorcist or the yokai.. 0.o either way it sounds very awkward..So I'd prefer Rikou to marry his own kind, well his quarter kind, Tsurara and (this is optional) he ought to marry someone who's strong too! Plus, Tsurara has been always there with him!!!! Besides I can't stand watching Rikou getting married to a weak yokai....
He'll most likely have a traditional marriage with Tsurara. After all, the last omake of the last volume showed his feelings for her when he tried to kiss her and the Nura seem to always have the traditional theme as marriage, since Youhime and Nurarihyon's and Rihan and Otome's was traditional (Rihan and Wakana's probably was too).
By the way, for the maker of this question, this reason seems weak. It makes it look like, just because his father and grandfather fell for humans, Rikuo is obligated to do the same. And for the record, Rihan married a youkai before marrying a human, so your theory falls apart. Sorry.
tsurara oikawa san or her other name yuki onna she is perfect for rikuo besides kana ienaga not so sure she i think rikuo does not like her look awesome right rihan and yamabuki rikuo is choosing tsurara