Heroes must be available for war
I've commented on other posts related to this before, but starting a new thread as I'm cranky. War is a brilliant addition to the game. Heroes are a brilliant addition to the game. Balance depends on it. Upgrading heroes is an absolute necessity to progress to high levels, and along the way to grind up the upgrades to walls and hero levels war is where the game keeps its excitement.
Heroes MUST be made available in war. They are too critical to the balance of high level battles, they are too critical to the team work. Supercell can't continue to hold out that the 200 or 300 whatever days of upgrades is acceptable to keep heroes out to get them up to max level when clanmates depend on you to keep them.
I don't want to sit out wars (which currently requires quitting clan if I intend to stay at dark elix farming range). I can't progress in the game without leveling heroes up. And if the result of this stupid failure of a feature is that I'm quitting my clan to stay out of wars while I farm hero levels, I'll be quitting clash.
I don't actually care how this is implemented. But some suggestions include:
1) An armourer building - the armourer (like the lab) simply upgrades the hero level or hero ability (or anything else about the hero: weapon upgrades, armour upgrades, fancy dress, any old dumb thing to do with a hero that gets people spending loot/gems/etc).
2) Treat hero upgrades like lab upgrades. That would be difficult or frustrating for some players, so consider giving them the choice to use lab time or put the hero down for the required period.
3) Include the hero in any war raids - that is, even if he is "sleeping" for whatever reason (upgrade or sleep recovery) simply have him available for war.