We’ve had 2 separate patches submitted in a relatively short timespan to allow Action Constraints to be controlled with a single float property input, rather than requiring a bone target. This is useful for a variety of reasons which have already been discussed at great lengths;
The actual functionality is very straightforward to implement but the UI/UX side has some sticking points (see discussion on https://developer.blender.org/D7572 and https://developer.blender.org/D8022)
As far as I can tell this is what we seem to be leaning towards as far as the general design goes, but perhaps there’s a more descriptive name than “Time Factor”
IMO this is the most intuitive way to do it, but people might prefer to have an unbounded input (not a slider) and set a start/end range on that. I’m not a fan of that design since chances are this property is going to be powered by a driver, and any remapping of values should take place there rather than in the constraint (it’s what drivers are designed to do after all).
I’d also like to just reaffirm that I had no idea there was a patch submitted already - mine looks like it follows a lot of the suggestions made under that one, but that’s a genuine coincidence (even down to calling it “Time Factor” haha)