From Kirei Cake:

Aisupe @ is a love comedy story based on the online game “ai sp@ce” that features various well-known heroines from popular games and shows such as D.C. II, CLANNAD, and SHUFFLE!

Taking place approximately 50 years after the original D.C. - Da Capo series, the story revolves around Sakurai Yoshiyuki and Otome & Yume, the two granddaughters of the original D.C. - Da Capo protagonists Junichi and Nemu.

This is actually the second manga based on Da Capo II in active serialization. The other manga, running in Kadokawa’s Comptiq magazine, is considered to be the “canon” adaptation of the story (i.e. it’s based directly on the game). As the title “Imaginary Future” suggests, this manga proposes an alternate deviation from the game’s premise, where fan-favourite Shirakawa Nanaka (speculated to be a descendant of the famous Shirakawa Kotori) is the story’s main heroine.