Now that I've got a little bit of playtime in, I can share some initial thoughts. :)
This game is quite challenging! I'll need a fair bit more practice to start building more than just menzen tsumo, tanyao, etc.. I found that receiving tiles in fixed triplets sometimes makes it tricky to access the center tile, especially with only straight-line melding to work with. However, having access to a Hold function seems very useful; I'll have to practice using it more effectively.
Have you considered also melding tiles in L-shaped formations (Puyo Puyo, China Town, Geki-Toride, etc.) or along diagonals (Columns, etc.)? That might give the player a bit more leverage while working within the restriction of receiving tiles in fixed triplets.
I like the idea of awarding additional fu for chaining. :) I think that's a nice way of rewarding some puzzle game actions while still keeping things integrated into the riichi mahjong theme.
Also, I have a question about special hands; from the included screenshots, I see that kokushi musou is implemented. Given the straight-line detection of other melds, does kokushi need to be built in a single 14-cell column of the playfield? Is there also detection implemented for chiitoitsu?