Leagues, special races, teams, etc. could have special adverts per server. Would be great, though how much load time would it add to every server you entered? I suppose you could turn it off like you can skins, and only turn it on when you want, like for that special race or league server.
Hmmm...that would require that none of the custom pics for any of the 100's of different servers had the exact same name, or your local downloaded pics would be overwritten. It would also place a lot of load on the servers when people connect. So, in that case....
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Now, if there was a way to allow downloading of a skin pack hosted on another server, like what some of the FPS games do with custom maps, then maybe that would be OK.