Here are my new theories based on the new material in the season 2 teasers/trailers so far. When I refer to 1920/1953/1986/2019/2052, I’m referring to them as timelines not specific points in time, in order to include events in those respective timelines that happen in 1921/1954/1987/2020/2053 and so on and so forth.
1. Young Jonas is trapped in 2052 after touching the wormhole created by adult Jonas. This is where he gets the hideous scar on his neck which adult Jonas has in season 1. (Perhaps it is an adult Elizabeth who saves him from being hanged.) An apocalyptic event occurred between 2019 and 2052 which left Winden a wasteland (possibly on the 27th of June 2020). Young Jonas finds (Claudia’s?) ad hoc time machine in the ruins of the nuclear power plant and uses this to try and get back to 2019, but instead he gets sent back to the 1920s.
Jonas becomes trapped in the 1920s, encounters young and adult Noah (who gives him Tannhaus’ broken time machine) and must then wait for the cave wormhole to open in 1953 by which time he is adult Jonas leading directly to adult Jonas’ arc in season 1 (once the cave wormhole opens in 1953, he can travel to 2019 and give his younger self the letter and lamp and give 1986 Tannhaus the broken time machine to fix).
2. Adult Jonas is currently (still) in 2019 since there are two scenes with him interacting with adult Hannah. Perhaps, he is there to try and stop whatever apocalyptic event happened between 2019-2053 and knew he had to send the younger version of himself 33 years into the future to continue the cycle. He teaches Hannah about time travel and tells Martha the truth about himself.
3. Old Jonas is a crippled, disfigured man known as “Adam” in the 1920s timeline. He leads the secret hermetic society who build the gates to the wormhole in the caves. Noah and adult Bartosz also seem to be members of this society. It appears he witnessed the apocalyptic event (as adult Jonas) which took place on the 27th of June 2020, survived but was severely disfigured by the event and travelled back to 1888. Perhaps old Jonas has recently passed away, and the casket young Jonas sees when he arrives in the 1920 timeline actually contains the body of old Jonas.
4. Young Helge is trapped in 1986 after touching the wormhole, and Noah is trying to send him back to 1953, so he can help Noah perfect his time machine as an adult in 1986. (Noah knows he will be successful otherwise adult Helge would never have been present in the 1986 timeline.) Adult Jonas’ wormhole destroyed the time triquetra, so 1986 isn’t connected to 1953 anymore. Time travel via the cave tunnels is impossible at this point in the timelines, otherwise young Helge could simply crawl back to 1953.
5. A young Noah will be met by young Jonas in the 1920 timeline. Jonas is the “stranger” that Noah refers to when talking to Helge is episode 10 of season 1. Adult Noah also appears to be active in the 1920 timeline and introduces young Jonas to the secret society.
6. Ulrich will be put into an asylum in 1953 and will escape in 1986 to try to kidnap Mikkel to take him back to 2019. He will be shot dead by police in front of the caves. This will be extremely traumatic for young Mikkel.
7. Hannah learns about time travel from adult Jonas and uses Tannhaus’ time machine (perhaps without Jonas’ knowledge) to go to 1953 and make sure Ulrich is locked up for good.
8. Adult Claudia from 1986 will dig up Tannhaus’ time machine and use it to travel to the 2019 timeline where she will see adult Regina undergoing chemotherapy and wasting away. At this point she doesn’t understand the cyclicality of time and will try and change the future. This will lead to bad choices which she will later regret as old Claudia.