I've just recently watched this movie a few hours ago. I wasn't planning to write any reaction or theories after the movie, but after watching it I just decided to drop all my reactions and theories. Warning: spoiler alert!

New characters from Harry Potter movie series introduced:

First and foremost, say hi to Nagini. Harry Potter fans knew Nagini as the vicious snake who has killed a number of important characters from the movie series, and perhaps, her famous kill was Severus Snape.
Nagini's first appearance was in the fourth movie, Goblet of Fire, and she was seen feeding from the poor muggle in his own house.
I think it's rather exciting to see Nagini's former human self before transforming into a snake. She's a friend of Credence the obscurus who possesses great power in the film Crimes of Grindelwald.

Secondly, Nicholas Flamel who's name first introduced in the very first movie of Harry Potter, The Sorcerer's Stone. The title was pertaining to the stone that can give immortality, which he later used, invented by Nicholas himself who's been very popular because of his genius creation.
YES he appeared in the movie Crimes of Grindelwald as Dumbledore's trusted friend in which he gave Nicholas's address to Newt if ever he'll be needing a safe house in Paris.

Thirdly, like what most fans have been waiting for and he's already part of the movie trailers.. Albus Dumbledore. A middle aged man in the form of Dumbledore appeared in the movie too.
I was quite excited as it was explained in the second film COG why Albus admires Newt, thus, answering the question of Gellert Grindelwald portraying as Mr. Graves in the first film Fantastic Beasts and Where to Find Them.
Albus came to Newt in the hope of asking him a favor to stop Gellert Grindelwald and Credence from creating a massive war between non magical folks and the wizarding world. Newt asked Albus why can't he do it, the answer was later revealed at the end of the movie as Newt held a piece of shiny object that was the blood oath of Albus and Gellert when the two were younger. They were the best of friends until Ariana, Albus's younger sister, died after getting involved in an epic duel between Gellert, Albus, and his brother Aberforth. Because of the blood oath, Gellert is protected from Albus and which he can never harm him. The very reason why Albus refused of stopping Gellert in the first place.

Lastly, Leta Lestrange, Newt's childhood friend and lover was also revealed in the second film. In the first movie, an old picture of her was seen by Queeny inside Newt's case. Then it was later explained that Leta was Newt's lover but she was a taker and Newt was a giver. It was not explained clearly how both of them had separate ways but Newt got an expulsion from Hogwarts after Leta's experiment went nuts and almost killed a life. Newt took her place rather than seeing his good friend getting kicked out of Hogwarts. It was later revealed too that Newt's brother, Theseus, won Leta's heart. She played a good part in the second film although she died from Gellert's hands. This provoked Newt to join the Aurors to find and kill Gellert Grindelwald which will be continued in the next movie..

I think it's gonna be another 1 whole year for the second part to be released. No matter, I'm waiting for it and counting for the part 2 to redeem itself from their fans.