Something with a prominent calendar and as much automation as possible.
Some european tournament sites lets players in a tournament upload replays which are then parsed and checked so don't have pm admins and admins are free to do other things. I think this kind of feature would be a big selling point.
I think i saw something on TL about a guy who made an open source php or java replay parser...
While I think some people don't like having to signup for sites it would be the best way to do it. It wouldn't be too hard to have users check certain flags and set a time to be notified of any tournaments matching their criteria (from prize money to league to time of day)
Allow anyone to add tournaments without the excessive oversight thats needed on TL calendar (until server load becomes a problem)
Featured tournaments as well would be cool.
Integration with streams and even allow people to register as streamers/casters so tournament admins could invite casters...
User stats would also be a BIG draw point. Things like win rates for maps, matchups, tournaments entered, highest placements etc. Same for casters, have tournaments casted and people able to upvote casters
I have been thinking about the stats idea with Clan FaDe's internal league but alas i'm still just learning C++
Speaking of leagues you should also be able to host them quite easily with the above features
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