Since I'm not seeing any content restrictions for ideas I'll take this opportunity to go all-out.
- Setting: Giga is resurrected from the dead and all the Waifu's are thirsting for him because he is the one compatible dude within the location, reincarnated there to be a literal stud. However, the Waifus don't want just anyone even if they're a hunk, so alongside the ritual a Dice tournament is about to be hosted where Giga must win in order for the resurrection to be extended indefinitely and win access to the Waifus. The point is a test of the resurrected's ability to strategize as well as his luck, with a preliminary period where Waifu players are limited to a single non-basic die-type in addition to basic so the man can get used to the game and earn dice for the tournament.
- Progression: Most Waifus have to have certain dice while beating them to make relationship progress, typically their own but sometimes a mix that may or may not include their own. In conjunction with "Setting" you cannot unlock full sex scenes until after you beat the tournament, though perhaps with a work-around where if you purchase/acquire a single-use condom-item you can see it. Limited progression should be based on having the right dice or at least beating the right opponent as well, such as needing to defeat (according to the current lineup and map placement) the Dragon Waifu, Kitsune Waifu, and Elyndra in order to be deemed ready for the tournament. Then during the tournament for each opponent you beat you receive their sex scene immediately if you completed the previous conditions as their way of "rooting" for you.
- Original Character: Kobold Merchant, trades in dice. Give two dice and receive one in return of any type of a Waifu you have beaten previously, receive a healing item (amount healed depending on dice cost), etc. Male but has a genital slit, in conjunction with the setting idea would be a former arrival who won but confessed to being gay and so has been left to his own devices. Appearance wise is a twink preferably on the slim side, thick-but-short stub-tail and scales instead of horns, with white scales primarily, soft pink speckles on the shoulders and thighs, and golden eyes. Can be challenged for the Exchange Dice, a 1/2/2/5/5/6 which when rolled allows you to select a die of yours to return from the board to the deck, and if used as the final die can potentially reverse a bust. Ideas for character/relationship progression will come once I know the general format and complexity.
- Hoard Dice: Possibly or a Mimic Chest Waifu, makes it so that the Win Condition is whoever has the most dice on the board. Values depends on game balance, but should create an interesting situation where both players now want to roll low values to force the other to Stand first. Best included if there is a mode where participants take turns rolling first, since it otherwise has niche use cases to not always result in a draw (not counting other dice abilities).
- Steal Dice: Possibly for a Rat Waifu, throw it down and you the opponent's lowest value die on the board becomes counted as yours. In exchange for the power this one should roll low or even into the negatives to prevent simply stealing until the opponent has no dice left to roll and winning that way.
- Buster Dice: Possibly for an Oni Waifu. This die wins if you bust with it, but if you don't you're forced to Stand. Because of this it should have a ridiculous range of values, such as (to be rebalanced): 0/3/6/9/12/15.
- Swarm Dice: Possibly for a Bee Waifu. When rolled die causes the next to be rolled twice. This one would be the hardest to balance, and would probably make it so that every other die need to be balanced around this ability (eg Buster Dice would need to be far less likely to win from a single roll to account for how much more likely they can win from a double roll). And it needs to be made that this occurs only to a limit of twice, lest you risk One Swarm Dice --> Two Swarm Dice --> Four Next Rolled Dice.