I don't know how significant any love interest for Beckett & Castle really ever is.
Will was set to be significant and that really didn't last two long on our screens.
Alyssa Milano's ex-love was said to be significant and she came and went in one episode.

Won't getting a significant love-intest be like repeating the Will story again. Isn't it going to tap into the same reaction from Castle reguarding him, similar to what happened with Will.

Based on Stana's comments in TV Guide we know that they are both going to date others and see how each of them reacts to the other dating. I suppose that Castle does do non-serious dating, while Kate isn't the mess around type. Perhaps something that happens to her in 17 & 18 effects her and them. There are still 6 episodes after 18. Usually things don't really play out as they appear, usually somebody who causes conflict for Beckett or Castle just shows up for a few episodes and then they are gone .. gone… gone..

What always amazes me about shows like this is, they will spread out the hookup of the two main characters in an unbelievable way and yet on the other extreme, if new love interests appear they mock spead them to serious

I think that both Will and Kyra were significant. Well, Will was supposed to be significant but Bailey Chase is on another show, so they can't really use the character like they would have. And Kyra was significant for Castle (and Beckett, i.e. "He's all yours") but she couldn't be in more than one episode–she did get married after all.

I like the idea of Beckett getting a significant new love interest. Even though we see Beckett and Castle get jealous at the thought of the other having a date, they don't realize they're jealous. And I think one of them getting into a relationship, not just a silly date like in "The Third Man", will make the other realize (or get close to realizing) the extent of their feelings for the other. After all, we always come to realize what we want when we don't have it. Plus, it'll make for some great tension if Castle tries to interfere in some way to keep Kate from going out with the guy; even if neither of them realize why he's doing it.

I was just watching so old castle episodes. I'm sure this has been discussed but I thought I would bring it up again.

In "Little Girl Lost"
Beckett sees Will at the scene, she leaves Castle's area and is zoned in on Will. Right away Castle notices this, picks up on how effected Kate is. Castle waits to be introduced to Will by Beckett as if he is an after thought. Skip to elevator, as we look in, Beckett is in her spot on the left and Castle his on the right (reverse if you are in the elevator). Beckett & Castle don't exchange words at first, it's awkward and then Kate blurts out 6 months, we dated for 6 months. Castle says I didn't ask, Beckett, yeah I know, you were not asking very loudly. Skip to the kiss, which Castle walks in on, sees. Later Beckett wants to explain the kiss and there is an awkward exchange between Castle & Beckett in which Castle lets Beckett off the hook by telling her he thinks he understands Nicky Heat better than she think (meaning that he understands Beckett and what happened)..Castle calls Will a nice guy near the end of the episode. Will & Becket worked on a kidnapping case in past, Castle and Beckett are working on this kidnapping case now. Castle is able to help Beckett save the child, unlike Will. Will tells Castle that he is jealous that I hooked her, Castle responds, what's to be jealous of, you couldn't reel her in. Will left Beckett to goto Boston…Beckett didn't follow Will to Boston.

Skip to "A Rose for Everafter"
The are at the crime scene, and Castle happens on Kyra, he has the same reaction as Beckett did when she say Will, Beckett notices Castle's reaction just as he did hers. Skip to the elevator, Beckett & Castle in the same spots they were in "Lost Girl". Castle, we met in college, we were together nearly three years. Beckett responds, I didn't ask. Castle replies, yes,you were not asking very loudly. Skip to the fact that Beckett also sees Castle & Kyra kissing, not in person but thru the photo. Castle explains what he did as just being kiss. Beckett calls Kyra real. Castle wrote a dedication to K.B (Kyra) on his 2nd novel. Castle just wrote a dedication to K.B. (Kate) on his latest book. My guess is kyra was the one that got away, Kate will not be.
Kyra left Castle to goto Paris…Castle didn't follow Kyra

"The Third Man"
The both go on Blind dates, she with Mr July and he with #3. While on the dates their actions mirror the others, neither are interested in the date and both are all about the case. Move to the elevator, they are in the elevator sharing date stories as they head out to share a burger and shake.

Now it will be interesting to see, if the show brings on love interests for both of them, will they once again mirror what the other is doing?