This is a fairly important suggestion.
Introduce the option to replay the already finished Campaign missions - all 34 Campaigns.
When completed again, where applicable for that Kingdom, provide the Epic rarity card of that type.
When it comes to loot rewards, I would exclude any rewards that were given by default on the first run, only Gold, Souls, and Gold Keys would count, nothing else, except for the Epic cards or troops.
To prevent farming or stockpiling of the resources, like specific Kingdom troops of Epic rarity, I would recommend implementing the option to have three ‘Replay Tokens’ or ‘Campaign Sigils’ available on a weekly basis.
Weekly tokens, if not spent, would not stack or increase when the weekly reset happens.
This means that you can replay any type of Kingdom Campaign that you want, same or a different one, but only three times. When completed, if that Kingdom has a Troop of Epic rarity at the end of the Campaign, it should be given in a form of an extra Epic troop or card.