Amazing webpage, I often use it for my dungeons when playing on Foundry VTT through the importer.
There are two things I think could be huge improvements:
1. Have hard corners automatically have walls that stop line of sight past them. Here is a link to a number of examples where I added the corner manually: https://imgur.com/ghXx6gY
2. Allow the grid to be half as large as it currently is (or perhaps even smaller?) for dungeons that were built by larger than average creatures. Optimally, this would only change the grid size (allowing 4 creatures in each normal square on the map) while still maintaining the placement of walls and doors.