This sounds like it may be a very good idea. I especially like the 'data hiding' approach you described, for reports etc (delegating til later/appendix).
But I'm having a hard time understanding the detail of what this would look like, or how it would operate. You say "timeline might be by "Concern (condition or sign or symptom)" or by "Activity"." but I'm unclear what that means.
I can imagine a 'timeline' where you can select any 'object' in your PHR (a visit, a treatment, or symptom etc) - and placing those items on a timeline. But beware - they will nearly always (or at least quite frequently) overlap. Perhaps that is the point you wish you convey?
Another problem with timelines - in general - is scale. Do you maintain a linear relationship between 'time' and 'inches'? If yes, for many interesting things, large gaps in time makes other interesting events collapse into an unreadable heap. If you say no, the 'time' aspect of the timeline can be somewhat misleading.
I'm still mulling over your idea, and would appreciate your clarifications.
Records for Living Support Team.