Secondary challenge goals.
When runners are trying to set personal records, they frequently run with a pacer. In dog racing the dogs chase an artificial rabbit. Etc. There are many metaphors that are appropriate here. The ability to superimpose a secondary, more difficult goal, on top of an existing "minimum" goal could be similarly motivational. Add a little carrot to the stick while pushing yourself past the comfort zone.
I posted about this as a comment on another proposal, but I saw that it was already closed so I wasn't sure if you'd read the comment I made there.
Sorry to just now reply to this ancient suggestion! Our current feeling is that this is not so necessary given that you can continually adjust how ambitious the goal is (using the Road Dial). Eager to hear if you think that doesn’t quite capture your proposal!