I think symbiotic is the word you want, but that would be excellent. 
Create fully tameable/domesticable Lynx and Bobcats. That would be just the right size.
On that note, I would make pain gradually desensitize to a manageable level over time, similar to the ways that smells do (but not quite to nothing). Once it has gotten your attention and alerted you that something is wrong there is no reason for it to keep going off, but I do still want it in the back ground to remind me that I should probably get around to seeing someone about my broken arm.
Since it says petty, I would like to remove all pollen allergies. Not so petty, remove all allergies from all beings.
The human body should be equipped with a system that produces some sort of code that describes exactly what it wrong with it. As in, is that pain in my chest caused by a heart problem, a pulled muscle, or something else? The ability to self-diagnosis would save doctors from having to run countless tests to figure out what’s wrong, prescribe ineffective treatments due to conflicting symptoms, or dismissing what appear to be minor problems that turn out to something that could have been cured, or at least abated, if caught earlier (I’m particularly thinking things like cancer).
All cats would know how to use a toilet, and will do so for all their … err … leavings. Also, they would not decide that playing with the toilet lever is fun.
Oh, come on!
Some things are impossible, even for God.
A swift and painless death to the guy who responded thus, “Point Fermen? Oh, were is that?” when asked for directions to Point Fermen.
Is that you, George?