We all know that clans have not offered the same amount of features as we have seen in CoC. Other than requesting and donating, and promotions, much else isn't offered by being a member of a clan. Clan battles were a good step, but not nearly enough. So here are some ideas I have.
Clan Donation Battles -end of each week, top donor gets a chest containing 2,500-10,000 gold.
Leave all special events available for friendly battles -draft challenge, random deck challenge, double-elixir, etc.
Gold Requests -make it an option to request gold rather than cards from your clanmates. We have all been at the point when we need gold for upgrades, not more cards. Request and donate more gold as you reach new arenas & leagues.
Add friends -I am honestly puzzles why Supercell hasn't implemented this yet. Not everybody wants to have to add their clanmates on Facebook. Its a simple concept, and we have seen it added in CoC.
In-Game Voice Chat -will allow you to easily talk to your clanmates during battle. Helpful for clan battles, or just help from spectating.
Revamped Clan Tab -Separate friendly battle results, chat, and requests into different tabs within the clan tab
Clan Level -Leveling up clan through donations and trophy count. Leveling up could improve quality of clan chest as well as unlock different friendly battle options as stated above.