I'm afraid it's impossible in the short term because it will be the changing of one of the base mechanics so, the risk of regression bugs will be high. TimeLine Slicer is extremely popular visual so, we have to be quite accurate with every new features that we implement.
The second proposal is unclear to me. Could you please describe it with details?
Evgenii Elkin, Software Engineer Microsoft Power BI Custom Visuals firstname.lastname@example.org