Honestly - the only thing i really miss among the default utilities is the hypothetical command "sortby" (and maybe it's cousin "uniqby") which could be used, for example, like this:
... | sortby cut -d: -f2 | ...
Or ... | sortby tr [:upper:] [:lower:] | ...
I know there are ways to implement practically any type of sorting by using flags to sort and a little bit of {pre,post}processing but having a "sortby" would be so much more elegant. You would have the full power of awk/sed/whatever integrated seamlessly with "sort".