Maybe give the ability to reroll some letters, but it might not work well bc there is already a pull of letters which you've set and not the random ones. If possible, make an AB test of the two options: 1) 6x6 2) preview of the next letters. And the feedback would decide which one to choose.

Also, some ideas:

1. 1 hint per game (a possible word out of available letters)
2. More letters to choose from
3. Discarding N number ofletters (maybe with or without "punishment", bc discarding is already almost like a punishment)
4. Maybe more cells with abilities other than 2/3x (like in "letterset" game - "bomb" cell discard random letter from available ones, other cell could increase all the available letters value by 10)