Literally the only thing throwing a romantic angle in Charlotte is the gag that’s used over and over. That isn’t to say there won’t be romance, I just think the idea that you’re certain there is going to be between Nao and Yuu is a tad… Well I’d be disappointed in that case.
I’d argue that Little Busters isn’t about romance. The character you’re calling the ‘main girl’ isn’t that in the sense of a romantic way at all. So I’m not sure this works to support your thinking. But even moreso:
I feel like you’ve genuinely misunderstood (Rewrite spoilers)Kagari’s role in Rewrite. She isn’t a love interest of Kotarou’s. She’s a force of nature. She isn’t supposed to be loved, the love that Kotarou feels for her is a lie. It’s okay that it’s a lie. It isn’t trying to be anything but a lie. He isn’t loving her, he’s loving the world. Rewrite is about loving ‘the world’. About loving the people in it. But most importantly it’s about finding your own way, not a universal one. Due to that I’d also say this supports the idea that Nao doesn’t have to be a main girl in a romantic sense. At least if you’re trying to argue the existence of one Key work means another must abide by it’s rules. Which in Rewrite’s case is a tad odd since I’m fairly certain Maeda’s influence on it, as compared to his influence on Charlotte, is minimal.