We now know the first 6 quests for A Ripple in Time. Once the last two become available, I will update this list. Good luck in your search for Celebi!
Update: We now know all 8 quests to catch Celebi! It appears that there is really only 7 quests to be done. Then, after catching Celebi, the final quest is automatically completed with more rewards and additional dialog from Professor Willow.
Celebi Special Research is here now! You can find the quests under the "Special" research tab and just like Mew, there are 8 groups of tasks to complete. If you have not completed the Mew quest line, don't worry. You only have to complete the third set of tasks for Mew to start working on Celebi. "Johto Festival" Field Research has also been released today. It features many tasks related to 2nd Generation Pokemon.
I will be posting more information as more details for Celebi and Johto Research tasks are released.
4th Generation Pokémon will be coming to Pokémon Go soon. When it finally releases, there will be 107 new Pokémon to catch. A handful of these will be obtainable by evolving Pokémon from previous Generations. In order to get a head start on completing your Gen 4 Pokédex, you can collect candy for the following Pokémon and choose the best of them to evolve.
I got most of this information from an article on rankedboost.com. My focus was which Pokemon's candy you need to save up to evolve into Gen 4 Pokémon, but their article goes into more depth. Take a look at it to see all of the baby Pokémon and other Gen 4 Pokémon as well as their stats.