In Hisoutensoku, it appears that the last few enemies you fight in arcade mode mimic the storyline of another game.
For example, Suika appears to only be encountered (or at least almost all of the time) as the final enemy you fight but only if Yukari was the second last boss. Mimicking the storyline of IaMP.
Suwako has the same relationship with Okuu, appearing only after her as the final boss in accordance to the soku main storyline and even Tenshi does the same with Iku.
It may even be the same for Remi and Sakuya and Yuyuko and Youmu, but I haven't looked into it so far, but the ones above are confirmed.