Your mileage may vary (I noticed that some people like voice acting even if it's poor quality), but I think it's critical for Skyrim mods to have better support for text dialog. Heck, in Oblivion before OBSE with Elys' silent voice plugin, you had to pack hundreds of megabytes of silent mp3s so your dialogue text wouldn't scroll fast as hell!
So concretely, what do we need?
- Display more than one line of text at a time
- Control the timing for showing text or better, let the player click through at his own pace
- Player replies of any length
- Control the font type, size, color, line wrapping
- I kind of liked the Morrowind dialogue system with its "hyperlinks", I think it could be useful for modders
- Control lipsync to give the illusion of speech
- What else?