Support multiple characters in one cell
Dutch crosswords have a strange custom of merging the letters IJ in one cell. The word 'BIJL' for example, will use 3 spaces, where the second contains both the I and the J. The letters appear in separate cells when not adjacent, and JI will also cover two cells like normal. Crosswords doesn't seem to support typing two characters into one cell right now, so for most Dutch puzzles, this will be a blocker.
The pzzl website, which I'm trying to make a wrapper for right now, tries to fix this by letting the user type in a I, selecting that cell again, and then type '+J'. It also has a custom keyboard which includes IJ as one character. A more intuitive way to me is to let the user type I, and J in two adjacent cells, but automatically merges them into one if this is detected and a Dutch puzzle is played. But this requires accurately detecting Dutch puzzles and might cause issues in the very rare case that a puzzle doesn't merge IJ.
Also noteworthy is that the character IJ/ij exists, but isn't used in any online puzzles as far as I know.