Add an option to remember your selection
Like if you visit a domain semi frequently, but not enough that you want to create a new rule, have a setting so it will automatically create the rule if you want it to.
Joseph Hillenburg commented
This should be manifested optionally as a app/URL -> browser combination.
Optionally the user could (should?) be offered the option to remember the selection for a limited amount of time.
For example, what if I want all links from my mail agent to open in Safari for the next 30 minutes? Etc.
You can do this under the Advanced tab in Choosy preferences. :)
I like this -- sort of like Quicksilver for domain names. ;)