Is there any rule or reason why you can't shuffle your discard pile during moments of downtime, in order to get a better shuffle once your deck is fully depleted? My understanding is that the discard pile is a space to which order and sequence don't apply.
To clarify: I do not mean shuffle discard pile back into draw pile, I simply mean shuffling the idle discard pile with itself, and still following the rules for when to reshuffle for a new draw pile.
edit: The OP was not clear whether it meant "shuffle the discard pile and put it back in the draw pile" or just "shuffle the discard pile". Now that's clear, I'm adding the following:
True, as the rules stand right now, the order in the discard pile does not usually matter (sometimes it does, see below). Theoretically, you could shuffle the discard pile at any time and just keep it away from the draw pile. The rules do not let you still do it for the following reasons:
The status of your discard pile might still change before the "proper" reshuffle, thus you might still have to reshuffle. Consider the following example:
You are going to play the Market, which lets you draw a card, so why not shuffle the discard pile while waiting for your next turn? Well, you might receive a Curse until then, so you'll still have to reshuffle with the Curse in it.
Reserved for future "discard pile" rules. Currently, there are no cards that do something, say, with the top card of the discard pile, so the order in it does not matter. Despite that, the rules still don't explicitly let you shuffle arbitrarily, because DXV likes to have as much room as possible for future cards. For example, one day he could design a card that does something with the top card of the discard pile.
For the sake of completeness, there are actually cards that care about the order of the discard pile: Watchtower. When you gain a card, you may reveal the Watchtower to trash that card or top-deck it. If you do, then the Watchtower expects to find the card on top of the discard pile. Consider the following:
According to the base-game rules, you only shuffle the discard pile when you need to do something with the draw pile (e.g., draw cards, reveal cards, etc) and there are not enough cards to do so. You cannot arbitrarily shuffle the discard pile whenever you want to, for a couple of reasons, including:
Thus, there is a general, fair rule, that makes sense as well: you shuffle only when there are not enough cards to do something.
You are correct that there are no published cards that care about the order of your discard pile. What I often do is shuffle it, leave it where it is face down. If I have to discard further, I slide those in and reshuffle, otherwise, I announce that this is my preshuffled discard deck, now needed as my draw deck.
From my personal experience - I am also sometimes inclined to shuffle my discard pile during other player's turns, but I refrain from doing so mainly because of two following reasons:
The temptation is strongest when I know that I will take the cards remaining in my deck through action cards that I have in my hand.
I rather pay attention to the flow of the game i.e. what others are doing in their turns (it pays out), and also develop quick and efficient technique of shuffling cards during my turn.
