Avvenire is an excellent epilogue. If you enjoyed the rest of Aria, you'll appreciate these last three episodes in the story.
I feel I should mention that although this is titled Avvenire, which means future, roughly half of it is flashbacks. That's not a bad thing, I liked all three episodes, but it was not what I was expecting. From what I had read I was expecting this to be about Akari, Aika, and Alice in their new roles as mentors along with Ai as an undine, and while there is some of that, Avvenire isn't starting a new story, it's a denouement. And while that's not what I was expecting, it's a very good thing. The end of Origination did provide closure, but it felt a little abrupt to me. Avvenire leaves me with that warm, happy feeling that I feel Aria deserves.