What i can imagine:
A custom "Demi God" game that sits in the lobby among other custom games, though it has some green writing or something flashy to distinguish it from the rest.
1 game is active at a time, which is filled up by guild members as well as adversary guild members.
When a game is filled, nobody else can enter it until the match has finished.
Once the match is finished, the lobby opens up again.
What you can do is instead of having the players get the rewards directly, you boost up or lower ranks of the entire guild based on the outcome of the battle. Or the player can simply gain rank points for their guild.
This could facilitate how a demi god and his guild gain standings in guild rankings. The players would simply win/gain points for their guilld as well as their standing in said guild. Challenges like this could be simply ignored if the demigod makes them too unfair, or not worth joining.
Otherwise, guild activity could reach over to realm,duel,rank league games were guilds can obtain an overall rank based on how well their guild members are doing.
I still feel like duel mode could be combined with league play in some way...this extra slot could open up to a guild menu, were the rankings of guilds are put in place. My solution here would be to simply combine the level based rewards of duel mode and apply them to the rankings. Encourage live play. Next, you could make AI duels come into play if noones online, what happens here is you can still improve your reward level, but your monthly rank is left unaffected.
Perhaps in place of the duels you could have "Guilds and Tournaments" This is could be a screen that includes a list of current tournaments that demigods have made, along with the general ranking. Perhaps having the top 3 guilds prominently displayed for all to see.
I just don't think basing the rewards of these custom games on aesthetic rewards is realistic....just look were epic gear has gone (pretty much nothings happened, with the lack of resources being able to be devoted to it) This would just further complicate the expansion of that idea. If your going to make any guild stuff happen, i'd keep it strictly based in stats.
I suppose ultimately, I'm veering more towards the addition of tournaments, though the assetts here look great, and could be used for a number of things (Snapshot making daily challenges, Realm editor tools, 'advanced' options for custom games)