I think a good idea would be to be able to write ux, cx, sx, etc. and for Duolingo to change it automatically to the letters with diacritics.
I have seen this suggested elsewhere the response was that it is not (yet?) possible (technically) within Duolingo, unfortunately.
A problem with this type of feature is that it would involve programming the letter x to work selectively as a dead key, which could result in legitimate uses of ‹x› accidentally being used as dead key uses as well. Also, since there are many, many different diacritics, it would take a lot of effort, and probably teach some bad habits, not to require specified diacritics (for example, ‹ux› would not be distinctively ‹ü›, ‹ů›, or ‹ú› etc without writing a different program for each language).