1. Use ~/.cache/youtube-dl/ for the downloaded youtube-dl on Linux. By default, youtube-dl-gui would place this file in the home folder which is not exacly nice. Also set the default video downloads dir to "~" (because of the previous change, leaving this empty would cause the video files to be downloaded under ~/.cache/youtube-dl/)
2. Add an extra "mp4+m4a 1080p(DASH audio+video)" video format that joins DASH viedeo format 137 with DASH audio format 140. Without this, downloading a simple DASH YouTube video would result in a video file without sound. Since youtube-dl keeps the video-only and the audio-only files, remove them and only keep the final audio+video DASH file.

I'm not a programmer and my modifications might not be good enough, but they work...