I was thinking that it would be helpful to some people to have a distance parameter for dynamic settings.
The idea of this parameter would be to track how far your brush has moved from the original spot you planted it. This would allow several things like force fade as well as size changes that would be less wavering like the speed parameter. This would also allow things like gradient made quick and easy (some people have been wanting this) as well as brush strokes that could resemble motion blur lines if paired with other settings.
Here is an idea of what it might look like.
I hope you consider my suggestion.
In Photoshop, this is called “Fade”, and while I don’t use it often, when I do, it’s saves a lot of headaches, and I’d love to see it in Aseprite too!
For some types of marks that need to change over their lifetime but need to be consistent in some regards, this is great. Examples would be rain lines (fading out over a set distance) and grass blades (growing thinner over a set distance). Using stamped brushes is inadequate for these since they still need to vary in direction, and may need extra controls from pressure (e.g. opacity for rain, or colour for grass).
In addition to the min/max values of the parameter being controlled, it takes an extra one, the fixed distance (in pixels) over which the controlled parameter goes from min to max.
If I am correct, what you are referring to in your last paragraph is like the other sensor threshold options. Is that what you are talking about?