I like the guildmaster having an ulterior motive, maybe even being corrupt. Gives the game some inner conflict, and helps to differentiate it. Maybe no one knows he's dying, and he doesn't want anyone to know that he's forging "fetch rare herb" quests to get ingredients for his life-prolonging alchemy. (Maybe all those rescued princesses are being turned into medicine, too.) Those quests are the stick; every month, you have to have looted a particular combination of items or game over. It doesn't have to be a sharp stick, it could be a minor thing that ramps up slowly as you play, just so long as it gives you a motivation to level your fighters.
But you don't want to send the same adventurer on too many herb quests or they'll figure out something's up. And adventurers that figure out that something's up have to have "accidents". So you don't want every quest to succeed: you want to have most quests succeed (to level up your adventurers for the required quests), but you want some of them to fail, so as to get rid of suspicious adventurers. (But not fail so obviously that it creates MORE suspicious adventurers!)
In terms of a quest mechanic, I like the puzzle idea. Maybe you have a quest-launching area with set of cards/tiles/whatever representing challenges (monsters, obstacles, locks, etc.) and can drag adventurers onto it with their own matching icons (so this adventurer has nine hearts; this one has three hearts but can climb a cliff and has three lockpicks). A few cards are upside-down so you don't have complete knowledge (but DO have plausible deniability). If you match every challenge, they'll come back with the loot (gold, herbs, princesses, etc.). If an obstacle isn't matched, then they'll come home empty handed. But for each *enemy* that isn't matched, it's roulette time and a random adventurer on the team dies. (Hopefully that suspicious meddler! But you can never predict...)