Intent. Specifically, Opponent's intent.
I'll give a short rundown about how each of those games show opponent intent, and how the player is allowed to react to it. Then I will write my thoughts on why I think this design is good, and why I'd like to see more of it.
Gloomhaven
Gloomhaven is a cooperative dungeon crawler board game. The phase starts with each player picking two cards from their hand. Each card has a top and bottom part, and each part can also default to basic attack 2 (top) or move 2 (bottom), rather than the text on the card.
After each player has chosen their card, each enemy creature randomly picks a card that defines their action for the turn.
Order of play is determined by initiative on the cards, and then resolved. When a player starts their turn, they have completely information of how the battle will unfold. They may have picked 2 cards based on some plan, but now they will see how the plan will unfold, and they can adjust how they use their cards accordingly.
So Gloomhaven makes the player partially choose their action, and then allows them to react to the Opponent's intent by killing specific enemy creatures, moving away from other, etc.
Pandemic
Pandemic is a cooperative board game in which players are trying to prevent the world from being overrun by disease, and also find cures for the 4 major diseases.
The way the game works is that there is an infection deck, which has one card per location on the map. At the end of each round, a number of cards are drawn from the deck, and a disease is added to that location.
Upon drawing an epidemic card (from a different draw pile) the cards in the discard pile are shuffled, and then put on top of the infection deck. This causes the same location to be drawn again. This doesn't only work really well thematically (infected areas have a higher chance of being infected again) but also works in showing intent. Players know which areas are at risk and can prepare to prevent outbreaks in those areas.
Into the Breach
A video game in which the player pilots 3 mechs pitted against an alien race known as the vek. A round starts by the vek moving, and then showing the intent of their attacks. The player can then fully react to that intent by pushing, pulling, killing and standing in the way of those attacks. Into the Breach allows full freedom to react to enemy attacks but not enemy movement, which honestly sometimes feels rather random and thus hard to prepare for.
Slay the Spire
Slay the spire is a video game with deck building as a strong core mechanic. During a fight, enemies will show their intent: Defend, power up, attack or debuff. Players can choose what to play accordingly, but there aren't many options. If your opponent intends to attack, you need to shore up some blocks. If the enemy intends to do anything else, you pretty much just dish out as much damage as you can.
Why Intent works
I once read an article about video game AI in which it said AI doesn't need to be smart, it just needs to be predictable. If the AI is too random, it will often look stupid. If the AI is predictable, it will look like it has clear intent, even if that intent isn't brilliant. It makes sense to the player that an AI will act with a certain intent, as long as that intent is consistent.
Intent, as shown in the above games goes a step forward and simply tells the player what is about to happen. This makes the AI seems even more consistent, even though it doesn't actually change anything in their behavior. But more importantly, it allows the players more choices, and more interesting choices, without having to have completely learned all possible actions the AI might take.
Take a PvP card game for example, like MTG or Keyforge. You need to be able to know, in board terms at the very least, what your opponent might do, in order to play around it. Don't play too many creatures if the opponent can board clear. Don't rack up too much aember if your opponent can punish you for it. If there was clear intent in those games (say an AI played your opponent) then you'd have a much clearer picture of how to prepare for the turn. Yes, this makes it easier, and reduces the effect of depth of game knowledge, but it also makes it more accessible.
Lastly, when opponents show intent, it presents a very clear puzzle to solve. When I used to play Warmachine, a miniature war game, I would say that when my opponent ended their turn, I was presented with a puzzle. A situation I have to solve, that my opponent constructed. They produced threats, objectives, defenses, etc. I need to figure out their intent from the board, stop their threats and achieve my goals. And possibly, present a good puzzle for my opponent.
Intent in PvP
Would it work to force players to show intent, making their plans more obvious to the other players? Some games force players to show intent to win, such as Inis instructs players to announce they are pretenders to the crown before they try and win. This forces the leading player to spend a resource in order to win, and also lets other players know they need to react.
But what if you had to show more intent, which actions you intend to take, where you might go. I'd be interested in seeing if this develops.