The Clan Castle is a big part of any battle. Players get to bring a few additional troops they receive from other clan members, these are donations. You can only donate troops, spells, or siege machines in Clash of Clans.
How about changing up the Clan Castle significantly. Let us mix and match what we donate, or let players donate gold, elixir, or even gems to other members of the group. Saving up 10 million for an upgrade is very difficult. If clan members could all give a friend 50,000 or even 100k, that would be great. Help someone reach an upgrade quicker, etc.
Again, we don't think Supercell will ever add this to the game, but it's one more thing we want to see at some point in the future. They could add limits each week, that way people don't progress too fast. For now, just use those League coins on upgrade hammers, which is better than getting some coin from a friend.