PROBABLY WON'T FIX:
Embedded rules language not updated when language is set (in Analyse, Game and Problems)
If new live game starts in background, "new game" notify OK but not first move.

"FreeBoard", re-using a lot of Board logic, but with SVG (empty) board + SVG (empty) reserves.
Will be used for variants with custom non-rectangular board (Hex, at least)
Or, with other board shapes (see greenchess.net for example)

Merge variants 1, 2 (3) into one with sub-variant selection when starting a game (New Game + vs engine + analyse)
E.g. Checkered 1 & 2 into one, and so on.
Need additional field in variant code... in a generic way (radiobutton or select or checkbox?)

#New variants:
Chessplode
Tablut

https://brainking.com/fr/GameRules?tp=128 Massacre Chess

CWDA : need game options (also useful at least for Monster)
PizzaKings https://www.chessvariants.com/unequal.dir/pizza-kings.html
https://en.m.wikipedia.org/wiki/Chess_with_different_armies#Pizza_Kings%5B11%5D_(John_Lawson)

Coin Chess
https://msoworld.com/product/chess-variants/
Background: Unknown inventor
Rules:
Normal rules apply except for the introduction of a coin (or counter)
Black starts by placing the coin on any unoccupied square. Play then continues with players alternating turns as in normal chess.
On their turn, a player makes any legal move but may not move onto the square where the coin has been placed. The player may move over (but not onto), the coin square.
The player ends their turn by leaving the coin where it stands or moving the coin to a different unoccupied square.
The coin can never be placed on an occupied square, and therefore cannot be used to protect a piece from capture
A player wins by checkmating the opponent. Note that the coin can be used to remove escape squares from the king.

https://www.chessvariants.com/other.dir/nemoroth.html :-)