After having gone through the campaign now countless times, and with the quest rewards be so bad that I feel they're a negative use of my time, why not introduce a new mode, called New Game Plus. In this mode, all of the maps get reset. Al of the enemies get anywhere from 30 to 200% increased health, as well as better cards to use. For example, a goblin in the Highway Robbery Mission would now have 12 HP (a 33% increase) and instead of weak blocks, they would be replaced with blocks of a higher value. All of their attacks would be increased in value as well. Bludgeon becoming Able or higher, etc. Perhaps instead of increasing stats, stronger monsters could begin to show up earlier. For example, all basic kobolds could be replaced by the stronger variants (the 14 HP spear ones). I feel this increased difficulty, while seemingly artificial, would provide a more worthwhile challenge than the quests, and could possibly reward better chests, items, experience, etc. Obviously this would prove too much work for the current team, but I feel this would appease the long term players, especially those who have already completed all of the quests.
I'd agree within the context of this game human interaction offers more complexity, challenge, and its generally more fun. Of course my affirmation is subjective, allowance must be made for shifts in perspective from a SP-only standpoint. The fact remains, some people don't wish to play with others for whatever reason, so creating or enhancing an environment amenable to that fact is hardly an extraordinary request.