I really like the idea of it, but I'm not sure how likely they are to implement it.
I personally use the zipped version because it feels like it launches faster, which is nice when I'm constantly relaunching for mod making. Though, with RCON and jarg's pr for broadcasting to a local port, a feature that only conditionally does anything and doesn't report so isn't unheard of.
If implemented, I strongly request an api be made alongside it so mods can add more than the basic marker features. I don't think the ability to control custom timeline events feels very base gamey, but alongside mods like recursive blueprints and game speed controls it feels right at home.
I think the main question is which events should the base game register? As well as which additional icons should they register for mod use if any?
I'm currently thinking of these events:
Death (of course)
When the player first takes damage?
Achievements
Victory
Player join/leave
Crashes?
Maybe with a buffer system--like repeated chat messages--to reduce spam/clutter, and depending on how verbose the timeline is supposed to be:
Chat messages
Destroyed buildings
There's also timeline descriptions which I can only think, if used at all, should match the notification type messages in chat (something researched or someone died) with a little timeout maybe matching the fade-timeout.
This idea really got my brain flowing even though I know I will never personally use it :P