Advice, in addition to all the other advice here:
- Connector line routing needs to be good (currently it doesn't really work) and if it were great then that would be a huge selling point.
- You should consider including a ton of "Cloud" (AWS, Azure, Google) symbols and icons out of the box. There are crappy ones available free from Amazon, that would be a start at least. I think "cloud architecture" is one of the biggest use-case for diagramming software right now.
- Not sure if you're using SVG as the native format but if not, consider it. The main reason I didn't upgrade to the new OmniGraffle was its so-so SVG support. Imagine a use-case where people collaborate on a drawing and then store the result in Git, maybe do some batch editing elsewhere, then need to get it back for more visual editing.
- If you want to get traction in the Enterprise world (the horror!) then you probably need a Confluence plug-in at some point.