Allow having several timelines opened in parallel. Requires some work on the current code to make guides and timeline preview attached to one timeline only. First step would be to allow several timelines in the same project (sharing the same project bin).
Brainstorming: one way we could implement this is to add a new clip type in project bin: nested playlist. When double clicking such clip, it would open a new timeline tab where you can add/edit clips.
This 'nested playlist' clip can then be dragged from the bin to the main timeline...
WIKI Project-File-Refactoring: https://invent.kde.org/multimedia/kdenlive/-/wikis/dev/ideas/Project-File-Refactoring/
[ME] added some tasks from below
possibility to stash/hide a timeline
Ability to Duplicate, Copy, Cut, Paste, or Delete sequences from within the project monitor.
Ability to Copy, Cut, Paste clips from one sequence to another.
Ability to have multiple timelines open simultaneously (Like in the picture in @emohr commment)
Ability to drag a clip from one active sequence's timeline to the tab of another sequence, which will trigger to open the other sequence's timeline and drop the clip into desired spot in timeline.
A sequence can contain another sequence as a sub-sequence, either by inserting the sequence, or drag and drop one sequence from the project window into the other sequence's timeline.