Along the lines of what Elbirn said is a good idea. We've already got a quest for a rare weapon. One-time quests for a shard, another for one nymph, another for one free upgrade, and another for some electrum (1k seems a decent amount, but it would depend on the difficulty of the quest). This way, people could get a shard and a nymph for their favoured element, or just stuff to help them along. It can be a bit of a bummer, for instance, for someone who is an absolute lover of, say, the death element, to have 0 death nymphs and 15 nymphs from other elements; the quest would let them guarantee that one for their favourite element. The nymph one might be a bit controversial, but the rest are fairly easily obtainable anyway, so having one-time quests for a shard or a small amount of electrum would be fine. One rare spin in the arena already gives you a rare weapon or shard; the only difference would be the quest is a one-time thing, and you'd choose the one you want, and one rare spin in gold or plat gives you enough electrum for an upgrade. (cash from 2 or 3 wins plus the upped card)
Other possible repeatable quest rewards (maybe 1/day or 1/week or something) could be things like extra oracle spins, or if that was deemed overpowered (due to possibly getting nymphs), just certain parts of the oracle spin, like free random pets, some bonus electrum, a non-nymph card, another FG prediction, and so on. (or even halfblood predictions, since they have names, too; ai3 and lower don't have names) Even an arena prediction of sorts might be neat; for instance, you might be able to see the hp, oracle card, etc of the next arena match you play, and have an opportunity to try and build a counterdeck based on that information. It wouldn't really be any more OP than a FG prediction, and actually less reliable, since sometimes you might not learn a lot. (seeing air mark and air pendulum could be quite a few widely differing decks, for instance, but seeing darkness mark and ghost of the past... well, you know what's coming :p)
Other oddball quest rewards could be things like "the next game you play, your entire deck is upgraded for that match", or "the next game you play, you start with -something- in play", where it might be a pillar, a creature, a permanent, or whatever, or "the next game you play, you get more hp, bigger mark, dexterity, or something like that", giving you one of the attributes of a halfblood or false god. The only problem with those is they're all basically variations on the same idea, that of giving you an advantage in the next game(s) you play. They could be switched up a bit, like one day the daily quest could be something like 'beat this FG and get +50hp on your next game' or 'beat 5 halfbloods in a row and win 500 electrum', or 'beat FG rainbow with an EM and get a free shard', or whatever; it could even be randomized from a list, and maybe even different for different people, as part of your oracle spin, for instance.
Foil art versions of cards would be another neat quest reward, and one that wouldn't be overpowered. Alternate art would be a way of pimping your deck out without giving ridiculous advantages to anyone. (hey, look at me, I have six foil fahrenheits :p) That of course requires someone to make some new art, but I'm sure some people would love to contribute some kickass new art. (just not me, I suck at art :p)
