Adam,
Extending contests hurts the designers because entries are more likely to be copied.
How about this,
When you see a direction that you feel great about, but also want to see it developed further, why not do this,
end the contest, declare the direction a winner, get the files, pay the designer
AND THEN
start a second contest
and as part of the second contest, share the winning file and specify that you want this contest to be about
taking this idea and developing it.
Pay the same for the second contest as you did for the first.
The results will be awesome; they cannot not be.
bonojerry