Instead of a big open space....
Instead of a big open field, clans should be able to build a clan city together. Clan elders can collaboratively build a village up from zero. The buildings can look slightly different from the regular village buildings and have slightly different properties. Clan members donate gold, elix, troops and gems to build the city. There are no barracks to train troops, which must be donated by clan members. Keep the attack concept and the troop donation the same as is suggested in this forum.
Clan members can buy gems to donate to the city, or to make resources or troops to donate to the city.
This would let Supercell make money and give it a ton more options for development than if clan wars were just a big open space where people dropped the existing troop units.
A big open space with only troops will create one main problem. There is no opportunity for game development. Players donate troops. That's it. It'd be like the arena fights in Castle Crash, which get pretty boring quickly. Clan Wars, instead, should give you the opportunity to build. After all, that's the compulsive factor that has made the game so popular in the first place.