I don't see how it would be that helpful. The event enumeration at the moment is useless for organisational purposes anyway, because adding an event in the middle of the sheet makes all the ones below get pushed back.
I don't like this idea. Having worked regularly on an event sheet 10 levels deep with more than 5000 events, this might sound like a good idea in theory, but in practice for anything complex it would be an utter mess.
Letters would be right out because it wouldn't last more than 26 events. For each level, you'd have to add another point to it. So you'd get something crazy like event 126.96.36.199.188.8.131.52.9. It would be harder to read than what's currently implemented.
I think C2 should use what CC has, lines that show levels. Even better, I think they should be highlighted as well when an event in its level is selected, making it easier to spot which one you're supposed to be looking at.
@Arima, In the case of 5,000 events you prefer this: 3012 -3013 --3014 ---3015 ---3016 ----3017
Over this? 32 -32.1 --32.1.1 ---184.108.40.206 ---220.127.116.11 ----18.104.22.168.1
The first has no meaning beside counting events. It has no purpose too beside telling someone he/she has an error on event #40,321. Your highlight idea is a good start/direction but it lacks the point I was trying to make - a mechanism which will show you how deep your current event is.
You also forgot my prediction - events will be able to fold in the future. With my suggestion you won't see the mess you mentioned in the 22.214.171.124.126.96.36.199.9 scenario (have you really gone so deep?)
I agree with @Arima, many users use very deeply nested events and instead of being event 9, it could be event 188.8.131.52.1.1. Not only is that less helpful, it also is probably too wide to fit in the event sheet in its current design!
Perhaps sub-events could just be separately numbered 1, 2, 3... at each nesting level?