Hello Jean-Claude 
Your suggestions gives me the idea that there could be an automation add-on. Rules, for example, based on filters, would automatically perform an action the first time the filter matches. That add-on would run in background.
For example, in your case, that would need a rule based on a filter on your ancestors, and the action linked to this rule would be to add the defined tag associated with the rule. A constraint associated to the rule (or the filter itself?) wouldn’t run the rule nor trigger the action(s) if the Sosa tag is already settled. Action(s): The action could, of course, be a list of actions. Their execution could trigger other rules performing other actions (e.g. - stupid example - any Person record tagged with the Sosa tag would be set private - it could be made by one rule, the Sosa tag rule, or in a second one triggered by this tag or by other criteria e.g. - less stupid - presence of the Sosa tag and absence of the Completed tag - another rule triggered by the Completed tag would unset the private flag).
Personally, when I create an Person record, I always add a citation, always the same one. This could be done by an automatic action (but I don’t know what filter/rule could be created to identify a new record and trigger the action — there are surely limits to this logic - maybe with a mix of filters and possibilities offered by SuperTool to access internal Gramps data).
I managed to do it with the CardView addon which allows you to detach a view and therefore see it at the same time as the Family Tree View (if you want to see what it looks like, I posted a video on the Discourse). More generally, it could be cool to be able in Gramps to detach views and continue to see them when looking at another one.