Clan Upgrades - Clan Bank
The idea is, there is a "clan bank" which clan members can donate to any amount of the resources they have it being eather elixir, gold or dark elixir.
Clan leader can then use those funds to upgrade the clan with similar upgrades as the level up system provides which ranges from the high tier donated troop level up to lowest tier 5% resources back when donating.
Cost for those upgrades should be very large (in hundreds of millions, even billions for highest tier upgrades) so its not easy to get them.
Bank should have a cap on how much resources it can fit in it and the cost to upgrade it should be very large too to make it even harder to get to the higher tier upgrades.
Also a history of donations would be nice so we can brag to our members how much we donated. :D