Timelines in MCU After Avengers: Endgame

There will be a Thanos from the past - the same Thanos we saw gather the Stones in Infinity War and die at the beginning of Endgame.

Steve returning the Stones to the past means that the timeline progresses as it ‘originally’ did. Thanos gets the Stones, kills half the universe, and gets killed, which leads to the Avengers gathering Stones from the past, reviving the dead, and Steve returning the Stones to the past.

As of the end of Endgame, there are four timelines/universes:

1. The main timeline. This is the one we have been watching since the beginning, and due to the nature of quantum time travel, no changes can be made - the past is the past.
2. The Loki timeline. This diverged during the botched attempt to steal the Tesseract from 2012. Thanks to Hulk’s interference, Loki got a hold of the Tesseract and teleported out of the Avengers’ custody, meaning he is free rather than facing Odin’s judgement. This is the point the Loki series will be starting from, and we will presumably see changes down the line as the result of the change. However, Thanos will likely go through with his plan as he did in the main timeline, but with one or more of the Stones being in different places when he arrives to claim them (the Tesseract being with Loki, the Aether potentially not being with the Collector).
3. The Thanosless timeline. This diverged when Nebula was caught by Thanos in 2014 and led to Thanos bringing his army forward in time to 2023 (where they were all killed, with the exception of Gamora). In this timeline, Thanos will not enact his plans because he no longer exists as of 2014. Otherwise, things would largely progress as in the main timeline up to the point of Infinity War, with the exception of the two Guardians movies (with Gamora not joining the team, Nebula not opposing them, and Ronan no longer being able to work for/betray Thanos.
4. The Peggy Rogers timeline. This diverged at the end of the film when Captain America returned to live out his life with Peggy Carter. We don’t know how things worked here - whether Steve allowed things to play out as they did in the main timeline, or whether he used his knowledge of the future to preempt problems - but Thanos would still exist to enact his plans, and the Avengers would still all be around… including the younger Captain America still frozen in ice at the time our Steve arrived.