I've seen a variety of reveals and ideas for gaff cards. I'll list some, with my opinions.
Double-backer - utility, useful, though not terrifically exciting.
Double-backer in two colors - see above.
Blank facer - I never found this as useful, though I'm told there are uses for it.
Barcode reveal - it's become a little staid and overused.
Tuck flap reveal - ditto.
QR code reveal - very intriguing, high-tech method for a reveal. Magician - or spectator! - uses their smartphone QR code reader app to unveil the code, a URL for a website that gives the reveal, whatever that reveal may be. Completely undetectable as a reveal until scanned. An alternate use would be to reveal a phone number, which when called plays an voicemail message that sounds totally normal until you reveal the predicted card verbally. (This would require setting up a voicemail number that only plays an outgoing message, but some magicians will do that and more for a great trick.)
Joker reveals - we've all seen the jokers where one has a card back in hand and the next one has the card face, giving a reveal. It's become too common. I like what was done for a few other decks, most notably the Arcane deck - the second Joker has the word "Joker" replaced with "3hearts" in a font style that conceals the change. I recently suggested one to someone who had "THE ARTS" beneath a joker head in his deck design: change the first T to a 7 and fiddle with the spacing a bit and you have "7HEARTS". These have a higher degree of subtlety but at the same time a strong visual impact on the spectator, who's now blinking his eyes to see if his vision is off somehow.
Court reveals - depending on how you execute your design, you can insert a reveal into the clothing of a court card's complex garments. I have some White Lions gaff cards where David Blaine does something to this effect. You could probably also do it to an elaborate Ace. It's not a bad idea to do such reveals on extra cards such as a duplicate Ace of Spades rather than the cards in the deck itself - it might be off-putting for someone to use, for example, at their poker table.
Utility card - This will sound silly, but it's true. One of my favorite extra cards in a pack was the extras included in NOC, NOC 2.0, Altruism and Altruism Snow Owl Edition. Although it was accidental in the case of the Altruism decks, each deck listed had either an extra Queen of Hearts or an extra Two of Hearts. There are a lot of tricks that rely on using two identical cards to pull something over on a spectator, one of the most common being the famous card switch - spectator holds one card trapped between his hands, you hold another, you wave your card around his hand, hocus pocus, you make them switch places. I find that I'm often doing this with Jokers, but most people already know that most decks have two Jokers. Doing it with a different card from the deck (perhaps even something more humble, like a 5 of Clubs or something equally as popular) will help sell the trick more effectively.
Other, more specialized gaff cards can be used:
The 11 of Hearts is popular. (You can increase the number to as high as perhaps 14 or 15 and change the suit as desired.)
Similar to this, though best used in pairs: the classic 3-and-a-half of Clubs (perhaps you could update it to a different value and/or suit?).
A card that has your back and a different Ace, such as the Bicycle Ace, is possible.
Bicycle Masters decks in red and blue often came with a Ghost Ace of Spades, front and back.
A similarly-useful card would be a double backer with your design on one side and a common design on the other, such as Bicycle Rider Back, Bicycle Maiden Back, Bee Diamond Back, etc.
A "smeared" card, smudged as though the ink was still wet.
A "broken" card, with the pips and indices knocked loose and fallen to one edge of the card.
A "blurred" card, intentionally printed to appear out of focus no matter how good your eyes are. (Great when performing for drunks!)
A "twisted" card, where the card appears to have been rotated too hard around a central axis, causing the image to twist and distort.
Double-faced cards - for example, a set of three non-spade Aces on one side, which becomes three indifferent cards when turned over.
Double-indexed cards - from one index corner it looks like an Ace; from the other, it looks like a 3 or a 2, or a card that's a King of Clubs in one corner, a King of Spades in the other (often paired with a similarly tricked-out red King with both red suits).
A "traveling pip" card pair, with one common example being a 4-card with five pips on it and a 5-card of the same suit with four pips on it. Diamonds tend to be better for this since there's no worries about one of the cards being rotated the opposite direction from the other.
There's so many more, I could probably write a book on the topic...but this should give you an idea of the variety of cards and reveals you can play with in your deck. Need more ideas? Poke around a magic shop, either online or in person, and see what they're offering.
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