Maybe simple implementation of subtitles (srt/ass) could be done by using the ffmpeg subtitle burn-in function: https://trac.ffmpeg.org/wiki/HowToBurnSubtitlesIntoVideo
So in Shotcut the subtitle-file-path would be added in the clip properties and is “burned” into the source clip.
[Maybe this is actually a MLT feature suggestion?]
The user-ediable-way would be when importing an srt file to automatically create a new track with clips with transparent backgrounds at the position and duration specified in the srt file and each clip should then have a text filter add on containing the subtitle texts, however the text filter would have to have a fixed text size option for consisting looking subtitles.
The alternative way would be to generate HTML files for each text and then added to generated transparent clips in a new track.
But I’m not sure this will be too slow to process all the compositing of texts and transparence?