One downside would be the risk of huge download lists which would put some off downloading when the route is done, so you could limit the number of downloads, say 20-30 or what ever you feel best appropriate per route.
The idea being considered is that every route would have to work straight 'out of the box'. This means that if third party or own assets were used, these would have to be packaged up with the route so that the user would simply download the .rwp file, install with the RW installer and then immediately start playing. No additional assets to obtain.
This way anyone with RailWorks installed, anywhere in the World, would be able to use the route exactly as built by the creator without any hassle. Getting that to work properly would be part of the Challenge.