The main reason I like Bitsy so much is because it is easy to pick up and learn. Someone can make a great game without learning anything about writing code, just using the visual tools. This is why I teach workshops on it - so many game workshops focus on code, and I want to instead focus on design. Having constraints can seem limiting, but designers deal with constraints all the time, and it often has the effect of encouraging creativity. Bitsy games are often small treasures where clever people figure out how to do stuff.
I would add the comment of adding in audio and even rudimentary speech support, but make sure it is done in a way that makes it easy to manage, and still works on anything that has a web browser. Instead of digital audio files, maybe have a simple midi playback engine that embeds the code into the HTML output. For novice users, managing external audio files with Bitsy can be a little confusing, so don't make that situation worse. Self-contained HTML output/input is one of Bitsy's killer features.
Modularity is tempting, but don't sacrifice simplicity. Bitsy strikes a nice balance now.