Customizable Visuals for parameters inputs for web service
If I correctly understood, as of today, parameters are limited to Power BI desktop templates (pbit). Only the Power BI Desktop users can be prompted, with a limited hard coded set of mostly controls based on datatype with hard-coded layout and displayed in a unique form. Power BI Service users cannot be prompted for inputs even if direct data style data connectors are being used. I am not sure, btw, if Power BI embed does allow the hosting app to collect parameters values and set them by code in the service on the queries.
There is a need for report providers, query creators, admins to be able to define custom "forms" and/or "controls" to prompt the users. The functionality should not limited to Power BI desktop but the parameters definition and the controls/forms should publishable to the service or web.
Some examples are:
Password style text box control ,
Numbers with units with convertor,
Complex types such as position (x,y,z)
Dependent parameters (value and display)
Hierarchy node selector "up front" lazily loaded as opposed to currently available full load/cached a posteriori slicer.
Graphic selector (why not map),
Non displayable parameter ( parameter defined by admin, but not available to end user)
Another option would be that a report could be the input of another report. In that case the Visuals of one report would become the gathering method for inputs of parameters of the second report.
The absence of such functionality is a real limitation for considering replacing or even just complementing an actual ad'hoc reporting tool.