I'm surprised that this isn't already posted as an idea.
The red-green palette can be problematic on a couple of fronts:
Deuteranopia -- the most common form of color blindness -- impacts the ability to distinguish between red and green (which then flows over to challenges with the gradient between those two colors)
"Green" and "red" are used for "good/bad." Color shading can also be used for simply "orders of magnitude" (for instance, traffic to a page: low traffic isn't necessarily "bad" -- it's just...lower; in some cases it may be bad, but not always). This can be hacked with someone by playing around with the custom thresholds and midpoint, but it feels a little hacky.
I'd love to see the ability to specify a color for the upper limit, midpoint, and lower limit in order to have more flexibility as to how the conditional formatting actually works.
To be really greedy, I'd love to be able to specify this at the Workspace level (or...even the account level...but palette settings already work at the Workspace level) and then override on a column-by-column basis.