BanG Dream! Movie: Episode of Roselia – II: Song I Am. Ending Explained [SPOILER!]

SPOILER ALERT: the following article contains massive spoilers, including the ending. If you have not yet seen the movie, proceed at your own risk, or better, come back to this article later!

SPOILERS AHEAD

BanG Dream! Movie: Episode of Roselia – II: Song I Am. Plot Summary and Synopsis

After overcoming numerous tribulations, the members of Roselia promised to appear on FUTURE WORLD FES.
It is now Autumn…

Through writing the lyrics of “Promise,” Lisa has matured.
Watching the change of color in the autumn leaves, Yukina and Sayo expressed their resolve in a picture letter to their future selves.
Driven by the wish to do something for Roselia, Rinko has started to compose a new song. 
“The remaining quests are the contest, the tournament, and FUTURE WORLD FES…”
Reflecting on their goals, Ako starts to feel anxious.

What view will the five of them see beyond FUTURE WORLD FES?

Source: Wikipedia (CC BY-SA 3.0).