We're currently considering adding a new game mode, "KoiKoi - Custom," that will let you customize the rules of koi-koi.
The customization options currently under consideration are:
Opponent difficulty: Normal / Hard
Number of rounds: 2 / 4 / 6 / 12
Cards in the starting hand: 6 / 8 / 10
Cards on the table at the start: 6 / 8 / 10 / 12 (6 or 8 when starting with 10 cards in hand)
Sake yaku score: 5 points / 3 points / Disabled (0 points)
Koi-koi declarations: Once (one declaration only) / Multiple (multiple declarations allowed)
Koi-koi scoring: Standard (if the player who was called against wins, their score is doubled) / Rising Stakes (the winner's score is doubled, regardless of who declared koi-koi)
When Rising Stakes and Multiple are combined, we're considering a rule where the multiplier increases with each koi-koi declaration: ×2 → ×4 → ×8 → ×16, capped at ×16 from the fourth declaration onward.
This feature is not planned as paid DLC. We plan to release it as a free update for everyone who owns Battle Hanafuda. Development is underway, with an early August release as our target.