I like this idea, and think it would encourage more clans/players to push for trophies. I would make a few slight changes though.
First, I would call it "Clan Challenge", and require a straight up entry fee. Next, I wouldn't make it a race to a certain amount of trophies, I'd just have it last for a certain amount of time, and whoever has the highest trophies at the end of the challenge wins. I also wouldn't start everyone at the same trophy count, or have players revert to their old trophy count at the end of the challenge.
Leaders should be able to decide what the entry fee is, either by entering a value for each resource on the "Start Clan Challenge" screen, or by selecting from a set of predetermined options.
Whoever is starting the challenge would also set a time frame, and the minimum number of players should be 10. Once the leader hits Start Challenge, everyone would get a message and have 24 hours to go to the clan game screen and join in the challenge if they want to play. If not enough players join in 24 hours, the entry fee is refunded.
Predetermined Options for Clan Challenge entry fee:
5 gems
10 gems
20 gems
50 gems
10k gold + 10k e
20k gold + 20k e
50k gold + 50k e
100k gold + 100k e + 50de
200k gold + 200k e + 100de
Players with the top 3 trophy counts at the end of the challenge would divide up the prize pool, with 60% going to the winner, 30% for 2nd, and 10% for 3rd. Alternatively, leaders could set number of places to be paid out, or the game could have a sliding scale that would increase the number of players paid out based on how many players participate in the challenge.
Another interesting twist might be an option to add some sort of handicapping based on each player's level.
