SPOILER WARNING FOR FATE/KALEID LINER PRISMA ILLYA, FATE/STAY NIGHT: UNLIMITED BLADE WORKS, AND OTHER FATE SERIES WORK
Ah, at long last I worked up the courage to finish watching Fate/Kaleid Liner PRISMA ILLYA 3REI!!!, and I must admit it's hard knowing that there won't be more episodes coming out for a long time. But the ending was beautiful, and I look forward to another season someday. They really left it on a cliffhanger, with all the events surrounding Miyu and her brother shrouded in mystery, as well as what Julius Ainsworth's next move will be.
It wasn't until recently, AKA today when I finally watched the rest of the remianing episodes, that I spent the time to completely listen to the ending theme song and watch the end credits. I was pleasently surprised, the outro song was very upbeat and got me very excited for more from the Fate series. I couldn't help but notice at the end, there was a scene of Miyu and Illya standing on a snow covered hill, with their backs facing the camera, which I have added below.
Notice that Miyu is wearing the dress that the Ainsworth's provided her, basically it was the stylish prison outfit they gave her while they kept her confined. She wore this the entire time she was kept under their care as a tool, and the Holy Grail. While under their care, and wearing that dress, Miyu believed that it was her fate to be the Holy Grail, and die for the sake of saving the world she lived in. She never questioned it, just accepted the fact that her fate couldn't be changed, she would have to die for the sake of the many.
Now for what Illyasveil is wearing, it the purple sweater type outfit that appears in 3wei once or twice. But more importantly, that is the main outfit that she wore in the Fate/Stay Night: Unlimited Blade Works anime series. In the UBW route, Illya summons Berserker, and ends up dying while in a battle against Gilgamesh in the Holy Grail War. She is a homunculi, her only purpose was to be a master in the Grail War and bring honor to the Einzbern name, as well as act as a vessel for summoning the holy Grail. Her entire existence, as well as her mothers, was just for summoning the Grail. They were disposable tools in the end, and if they failed at their task, they were useless to the Einzberns. They weren't even really people, they were in a way advanced golumns created by magi. Illya died, in an inescapable fate, knowing that she was just a tool, and didn't try to fight it.
I believe that this last, final scene in the series is showing the two previous versions of Miyu and Illya, when they were trapped by a fate that they believed couldn't be changed. It's who they were before they met each other, when they had nothing, no hopes and dreams of their own. One of the unique features of the entire Type-Moon and Fate series is that there are multiple routes and versions to different stories. For example there's Fate/Stay Night, Unlimited Blade Works, and Heavens Feel. And Fate/Kaleid Liner Prisma Illya is no exception, it's an alternative route to a normal ending where Illya and Miyu wouldn't have gotten to live on.
Prisma Illya, the entire series is based on a reality where Miyu and Illya met, and they were able to obtain happiness and move beyond what they assumed was their predetermined fate. The last scene is showing who they were, and the destiny that could have been shoved on to them, while the series is about them defying that and taking their future into their own hands.
This has been such a wonderful series, and a refreshing change from the normal grind of anime. While it does contain minor amounts of blood and a heavy, emotional plot line like the rest of the fate series, it does not contain that complete tragic tone. It's the alternate good end, and while it is a roller coaster and is incredibly in depth, it focuses less of the hopeless tone and loss. It gives more thought to the pure sensation of friendship and youth, and the joy it can bring.