EDIT: if there is a tip jar or a paid tier for the game, I'll be very happy to chip in!
Are there paid features you would be interested in? Would you want to buy a sticker or other merch?
Someone in another comment mentioned that they do a timed challenge with friends. This would require signup etc. Even though I sometimes do a race against my son I'd probably not sign up/register just to do this. But if it's purely optional then I think there is some merit there and people who are signup happy may take advantage.
I may also like to see perhaps a second (premium) daily challenge. Not unlimited mode because that would just make me gorge on it and get fed up. But two daily challenges instead of 1 might be something to ponder. Maybe even for a small pay? Just don't make it obnoxious with nags and popups etc.
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!"
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.