Hi all,
Thanks for your help. It's a good idea but still have some problems:
1. Your idea focus on Activities, not on target Entity (Account/Contact) --> I need to create a workflow for each Activity type (Email, Letter, Phonecall, etc...) with duplicate mechanism.
2. "Link" here is not only "Regarding", it means all Activities that are shown up Contact -> Activities navigation.
For example: The activities created by Campaign Activities Distribution does not have Regarding to Contact but it's also shown up in Contact -> Activities navigation panel.

Do you have any other workaround?
Do we really focus on Activities instead of Account/Contact?
If yes, do you know the criteria to show Activities in Contact -> Activities navigation panel?
Many thanks.