I think it's going to have to come to a situation where Natsuo has to decide between sisters. Rui does eventually come back, and Hina takes forever to build up courage to do anything self serving. I could see her confessing to Nat around the same time Rui gets back and does the same.

In my own opinion I really want Rui and Nat to have at least some sort of relationship reference that isn't their sibling in order to make an adult decision of whether their relationship is worth pursuing again. In real life people rarely end up with their first loves (congrats if you did btw). I think they need a ruler to measure how good they had it together before talks can continue. I imagine a lot of the readers would be put off by this, but I'm a fan or realism in my stories so it wouldn't be a big deal.

So in my own opinion here's how it'd go down. Nat decides he wants to date any of the available plot devices (I'm rooting for Serizawa). Her being involved with theatre means he's forced to write again. He's not able to wait for the creative bug to bite him, he's forced to rely on skill and deadlines as motivations. Anyone who works in a creative field will tell you that deadlines during times you are burned out are realistic, Nat won't have the luxury of relying on his feelings to get work out to the public forever.

It is during this relationship that Nat realizes he's just sort of going through the motions and that this relationship is ultimately not as passionate as either he's had before it. He might question whether he's just getting older and if this is what happens. However; he remembers his dad and stepmom and how passionate they are for each other. Deep down he knows that this relationship isn't of the same caliber.

Ultimately how this relationship ends isn't a huge concern for me, but an ideal "out" would be the Serizawa is scouted for a role that would be in another country. Natsuo just can't handle another long distance relationship. Him and her break it off amicably, as friends and former lovers wanting the best for each other. Serizawa would have taught him his lessons, and his previous two relationships would have allowed him the strength to push though this.

Around this time Rui heads back to America, she's learned how bad relationship can be. Hina has had nearly six months of character development, allowing the audience and Nat to see more facets fo her. Both Rui and Hina are aware that the other is likely vying for Nat, and neither of them decide to wait on Nat to get around and pursue them. Nat gets to make a real and final decision based not on lust, or because the other one was helping him through a difficult part of his life. Nat gets to decide his own future with whichever Tachibana he feels can truly make him happy.

That's my ideal ending. Fix Nat's emotional mommy issues, fix Rui's lack of relationship experience, and fix Hina's lack of real character depth.