Any cards, mechanics, and ideas that would work great in digital format, but would be impractical to do IRL? This can include too much bookkeeping, stuff that would be too cost prohibitive, or otherwise impossible with the physical version.
Oh, one thing we actually did have was unverifiable claims. Namely, Throne Room that says you need need to play an action card from your hand, but didn't have to reveal your hand to prove otherwise. IIRC, another card was like this, probably also touched up with the 2nd edition wording.
As for other stuff...
1) cards that are assigned variable attributes and text on variable factors. For example, what turn # you purchased them, how many Provinces have been purchased, are left, how many empty piles are left (City and Poacher deal with memory issues by just going with how many current empty supply piles there are, not when it was purchased), how many coins you paid to buy them, how many +Buys or +Actions you had, how many cards you had left in hand.
The card text can be written with the appropriate text at the time of purchase.
2) Specific cards that do extra things. In an IRL game, you could take a sharpie and mark a card (making this change permanent, but work with me), or slip in a piece of paper if you sleeve your cards. These cards have extra bonuses or penalties based on some other ruleset governing them.