These are fun ideas!
The custom color theme is really interesting. Are there specific things you'd like to do with the custom color theme? That seems like a fun feature to build.
Having a second daily challenge would be great, but a lot of work. When I launch player puzzles I'm hoping to roll some of those out along with the daily puzzle so that on certain days you get a "bonus player puzzle!"
On custom colour themes. I have no good ideas. I just know when I like a theme and when I don't. The deafaults in the game, are.... not great for me. Especially the light theme (I generally prefer light themes). I use the dark theme in the game now because light is not that great but dark is also a bit ho hum. So maybe a theme store would be good? A buck a theme or something? I'd rather not do theme imports etc. Too much work and I don't know what I'd like. I'd rather just swipe through a bunch of themes and hit "buy/pick" when I find one I enjoy. That's all.
One small feature I'd propose is that maybe you want to have a "hard mode" baked in where no hints are available? I personally do not use hints to solve the puzzles. I can usually piece everything together just with what's on the board and the word descriptions and never reach for hints. Hints feel a bit like a cop out so I avoid them. Maybe having this as a different game mode is a good idea? I'm not sure - it depends on the complexity of your implementation of you want to create a feature flag or not.
Great feedback, thanks!