I can’t get over any thought of how like, close and intimate a ChikaDiaSeira relationship would be. Like... Seira’s super used to people like Chika, loud and bright and vibrant if only so people will see her (after growing up with two older sisters that are noticeably better than her at academics, work, etc.) but Dia, despite having known Chika since they were young and having been friends with Mari for almost as long, still isn’t quite used to her personality so Chika spends a lot of time just hugging Dia and calling her “Dia-chan” or just “Dia” just to get her more used to the intimate closeness, and it just goes even faster after Ria and Ruby start dating and the Kazuno siblings start coming to Numazu more often to help Aqours train (and so Ruby and Ria can go on dates) and Seira, Chika, and Dia just spend their free time cuddling or working on future Aqours songs and maybe a couple other Saint Aqours Snow songs.
Also like, Ria and Ruby seeing how openly loving Chika and Seira are with each other and Dia and trying to replicate it but just being too shy and embarrassed to effectively say anything any sort of romantic without blushing like tomatoes and getting quiet for a few minutes.