Just a couple of “would be nice” suggestions.
Analysing a plot, character arc, relationship arc, etc., it’s generally necessary to focus on some particular element of a plot, character trait, relationship dynamic, etc. These are potentially limitless, and it would be useful to have some sort of marker to make these clearly visible and to be able to turn them off and on, preferably with a single click via a side panel or similar. There may be various ways of doing this - and the 2-d graph idea suggested in the old forum for the Subway view would be extremely useful in this respect. But another way would be to have a number of colour profiles. So in colour profile A, different colours might represent different characters. You might add a profile B where colours represent tension states, and profile C for power balances or broad sections of plot, etc., etc.
I would also vote for some sort of highlighting to make certain events particularly visible, and ideally this would be saved as part of a colour profile.
Obviously, there are lots of ways in the timeline to track all these things, but the point is to be able to represent them visually and immediately, and to be able to switch the timeline from one mode of thinking to another very quickly.