You can play Fione's route till the end. If you familiar with G-Senjou no Maou, the route is just like that. It's the 'what if' situations.
EDIT: And no, it's not the end of the game.
I usually play for 100% completion (even the bad ends like in FSN and Tsuikihime). But sometimes, I skipped some routes like in Canvas 2 (I skipped the little sister's route because she's annoying as fuck).