Basically I would love to have a configuration setting that turns the unfilled part of the bar "hotter" or "cooler" should I (respectively) gain or lose haste while attacking.

The feature's value is probably be clearest for fury warriors, where both mainhand and offhand can randomly produce(/fail-to-consume) Flurry charges. This immediately (un-)hastes the other swing timer's remaining part... ie, sudden change to some unknown fraction of a swing occurs frequently at random intervals, which perturbs expectations for both time-to-next-attack and your attack time's synchronization relative to the other weapon timer.

I, at least, could use more indication when that occurs... "hello, the rest of the bar is filling at a different speed, now". Changing the opacity/color-intensity of the bar's remaining time region is what I'm envisioning (eg, if the unfilled part starts filling up faster it also gets more red), but anything that allows conveying such info visually would be great.