With Alliance war in place:
Most players have or are working towards 60 heroes.
What about running a weekly Titan challenge event for all individual players.
A Titan will spawn that only you can hit. Once you use a hero it will no longer be able to be used. You get 3/45/6 flags to score the highest score you can and the leaderboard will determine the prizes like the challenge events.
You could have highest combo award
Highest individual hit and highest combined hits.
Titan star lvl and colour will be random.