A certain other online CCG is planning to introduce challenges, which will be several one-time series of quest battles that each share a common theme: e.g. ambush: the opponent starts with some creatures already out, and trap: you start the game with only half your HP.
Frankly, I think the idea is great and should be emulated in the daily quests. It gives you a respite of doing the same thing you'd do anyway (beat standard enemies, win cards, etc.), while not merely placing a "don't use X" limitation on your deckbuilding, but rather by introducing different game conditions that make it necessary for you to adapt or rethink your game strategy.
While I feel that ambush and trap would fit in without changes I'm not a fan of copying, so here are a few I came up with just now.
X Elemental Node:
Both players start with Y extra quanta of element X.
X-Y Dissonance:
Cards of elements X and Y have thier quanta costs doubled.
Challenge:
Opponent starts battle with weapon and shield.
Song of Doom:
Opponent starts with an immaterial nymph on the field.
Forest of Lost Memories:
Each player discards a card at the end of every turn.
Lord of the Depths:
An inundation effect is in play for the whole battle.
Lord of the Sky:
Both sides (must) have 6x Wings in their decks.
As for rewards, I'd like to see something you can't get by grinding. Like a couple of new ultra rare cards. My idea would be that quests fall into different difficulty classes, and get harder (with a better reward) each day until you either fail to solve one or "complete a chain", getting the highest reward. The highest reward would be a "useless" card that could be traded for one of the new ultra rares once you had enough of them.
