As far as the broader configurability questions, it's not so much that I want options libinput doesn't have (though that's a little bit of it).
It's more that I want the default behavior to be good enough where accessing the options aren't necessary (e.g., Macbook). In the event that the default options aren't that good, I want visual settings to tune the experience. Gnome 3 with libinput currently provides two options. Neither is pointer acceleration.
Since much of what I hope to improve the "feel" of default behavior, I don't think this could be as straightforward as "submit a PR to libinput." I think it could end up being a lot of work to get the default experience feeling great. The number of folks with interest in this esoteric subject seems to me like evidence of that.