Great finish!!! although it still hasn't explained some things but I guess they will be in the main series. 
I agree that's one of the few things I've seen people complain like oh we didn't get to figure out how Mavis died or why Precht turned evil, how Yury died, and I'm like that's what the original series is for. Mavis is still somewhat alive so it only make sense that we find out there otherwise it just takes the mystery away from the series.
I enjoyed Zero. Unpopular opinion, I was happy that this didn't show anything significant to the main story. Otherwise, I'll be like with Naruto: collecting treasures all over the map.
Couldn't have said it any better myself, that's one of the few things that makes Fairy Tail Zero work while Gaiden just relied too much on the older characters. Here we didn't see Natsu, Lucy, Erza, Gray, Juvia, Gajeel, Levi, etc cause it isn't their story it was a story on the first guild master Mavis for obvious reasons. The only big character that made a small cameo was Zeref, but he didn't really overshadow Mavis's shine.
Now here are a few honest questions I have to ask you guys, What were your thoughts on Mavis before you read Zero? What did you think of her afterwards? and did reading Fairy Tail Zero make her become one your of favorite characters?
I'll have to say reading this spin off made me give a huge amount of respect for Mavis's character, and I think she's definitely in my top 10 for sure.