Walter Montego: Well, Fencer recently released 8 new checker variants. Four of them are in the bottom 10 of the number of currently running games, and 7 of them are in the bottom 25. Even the most popular one, International Checkers falls out of the top 60.
If you look at all checkers variants, none of them is ranked in the top 25 of the current number of running games - not even checkers itself. And 6 out of the bottom 10 of number of running games are checkers variants. Almost all of the checker variants tournaments I organize are cancelled because they can't attrack enough players.
If you only look at the numbers, checkers would be the worst category of games for Fencer to put an effort into.
OTOH, if you manage to find a checkers variant needing dice, it has a good chance of become a popular variant. Out the 10 most played games, only Chess (3) and Five in Line (8) do not involve dice.
The short rules of Thai checkers mention that a capturing queen has to land on the square directly behind the captured stone. But this rule is not mentioned in the full rules.
coan.net: As for an easy variant to add, if Draughts ("International Checkers") is created, is Canadian checkers. It has the same rules as Draughts, except that's it's played on a 12x12 board (with 30 checkers).
Wikipedia has a handy table which lists the differences between the regional draughts/checkers variants, and has links to some "invented" variants.