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If you directly challenge another user, will that game be ranked? I can see a lot of situations where people would like it to be ranked (as an option).
The problem is that I don't see a good way to prevent someone from "Dumping" Elo points to another player by surrendering every game.
Based on how other games do it (SC2, LoL, HoN), "custom" games (i.e. direct challenges) should not affect your ranking. Can you still collect XP? What about achievements? Should there be a "custom games Elo" that doesn't show up on a leaderboard but that you can view for yourself?
I think the best thing to do would be to exclude custom games from any kind of ranking(Both xp and Elo), but achievements should be fine as they are more "personal" and you only would be cheating your self be doing anything.
Yes, exclude custom games. Ranked games should only be on certain "approved" maps as well.
@Kurshu I see your point about "approved" maps. But Matchmaking games should only happen on maps with a reasonable number of up votes anyway.