Motivation
In my opinion, seals are currently a very interesting mechanic in Balatro, and I would love to see them expanded upon in the upcoming gameplay update. This group of suggestions was also motivated by the fact that all current seal colors have corresponding stake colors (e.g. there is a red, blue, purple, and gold stake), but not all current stake colors have a corresponding seal (there are no green, orange, white, or black seals). This suggestion also capitalizes on this fact to fill in those missing seal colors.
New Seals (and corresponding Spectral cards)
Green Seal: When this card is destroyed, create a random Spectral card.
Tarots and Planets have corresponding seals, so I think it makes sense that spectral cards should have a seal too, albeit with a more difficult creation condition, to balance out spectral cards' more powerful effects.
Recursion (Spectral Card): Add a Green Seal to 1 selected card in your hand.
Since each seal has a corresponding spectral card, and (I believe) spectral cards are not based on any real-life card types (like tarots), it only makes sense that new seals would get new spectral cards. I'm calling this Recursion because Recursion cards can technically, with some luck, produce new Recursion cards, with some indirection in between.
Orange Seal: If played hand contains a scoring Orange Seal, gain 1 Discard (for this round).
It's important to note that this seal would not be affected by card retriggers, and playing multiple orange seals at once would also not have any additional effect. Only 1 Discard can be gained per hand played.
Enlightenment (Spectral Card): Add an Orange Seal to 1 selected card in your hand.
Couldn't think of a better name for this one, so I'm just suggesting this as a placeholder name for now.
Black Seal: When this card is destroyed, add a copy of it to your deck and draw to hand.
One interpretation of the color black is death, and potentially necromancy, so I'm drawing inspiration from that here. You're basically reviving a card with this seal, or preventing it from dying to begin with.
Necromancy (Spectral Card): Add a Black Seal to 1 selected card in your hand.
This one's name should be pretty self-explanatory.
White Seal: When this card is drawn to hand, draw 1 additional card.
White is the main color of (most) playing cards in Balatro, so I think the white Seal should have an effect that involves other cards. Note that this seal would (temporarily) bypass/increase your hand size, similar to The Serpent.
Manifestation (Spectral Card): Add a White Seal to 1 selected card in your hand.
Since my suggested White Seal draws more cards for you, you can think of it as "manifesting" more cards.
New Jokers
Seal the Deal (Rare): Retrigger all Seal effects.
I would like to see more Jokers related to seals, and this seems like a very interesting one. This Joker would be copyable with BP/BS, in line with card retrigger Jokers. I made sure that all of my suggested new seals had an active effect, instead of a passive one, so that it makes sense to retrigger them.
Here's specifically how each seal would be affected by this joker:
Red: One extra card retrigger
Gold: Gain another $3
Purple: Create an additional tarot card
Blue: Create an additional planet card (the same way Mime already works on Blue Seals)
Green: Create an additional spectral card
Orange: Gain another discard
Black: Create an additional copy of the destroyed card
White: Draw another additional card
Conclusion
I think these seals and this joker would add some really interesting new potential builds/synergies to the game, which is something I hope to see in general from the 1.1 update. I hope you enjoyed reading these ideas!