Well I posted a mode's suggestions here a while ago but it got rejected cause people thoght one mode is enough.
Anyway Now that were all more open minded I'll post the one's I think can work:
King of the hill- can definitely work in demigod and be quite fun.
Chain linked capture- like onslught in ut2004 and other game, more of a team mode than others.
Cart controll- theres a cart in the middle of the map and the target is to get it to your base by standing near it.
I like your idea, just it seems that it will be very difficult to get the flag back.
Oh did I said how happy I am for giving us more game modes, totally going to raise the life-time of the title