I see why people would say "A Song for XX", but I personally feel there are more appropriate songs that could serve as a memorial. Whereas ASFXX is about her past, her childhood, it's something she sings as a way to express her frustration at a certain point in her life, not timeless.
I had to think long and hard for this one. But after looking at some translations, I'm going to have to agree and say End Roll is a really excellent choice, lyric wise. So peculiar how she has seemingly so many sad songs, but only a couple fit lyrically.
Take a look at these as well:
too late - More of a sort of "life's too short" message... some people go through their lives at full speed, having all the wrong priorities and not slowing down to appreciate it. I didn't choose this because it doesn't describe Ayu herself and could be misinterpreted. The sound may be too "pop"-like.
a song is born - This would be my second choice. She wants us to remember the form this world should have. She wants us to appreciate life; that dreams and hopes are all in this world.
"Hold the flowers to your heart,
and pray that they may some day bloom."