I always found scrubbing very intuitive for editing (like on Waveframe if anyone knows it). With ardour2, I already find the CTRL-arrow forward and reverse play really cool. What about adding a slow-speed forward/reverse play controlled by arrow keys, e.g. with another modifier key (alt?) ? In terms of audio playback, the shuttle control already seems to provide the right function, it’s ‘just’ a matter of binding it to keys.
What would also be interesting is mouse controlled scrubbing with the pointer over the waveform display.