If you have some proposal for functionality in Bio.jl, then absolutely opening an Issue is a good thing to do - we can tag it properly and so on. But it may be helpful to open a thread here pointing to the issue, especially in the idea generating and spit-balling stages... as these posts will see a much broader part of the julia community and so may get more views and so more opinions and comments.
@kevbonham As a guideline I'm thinking of putting in our contribution guidelines to first post proposals here on Discourse in the very very initial discussion stages - especially if you have an idea, but aren't quite sure about it, want to talk to people first and so on. This will typically happen with Biologists and students unused to coding or new to julia, who might be nervous/unsure and may need a welcoming voice(s) to talk about their idea(s).
Once such people are more confident in their idea and sure they'll commit to the idea / proposal, then they can open an issue / PR. If someone already has a solid plan about their idea/improvement then of-course they can just open an issue or PR right away.
It's easy to just dictate X goes here and Y goes here. So I think ideas and proposals can be aired here for feedback if someone is not ready to make an issue or PR that is under stricter scrutiny.
EDIT: @kevbonham This very thread is evidence that taking the attitude I outline above is effective: You were not quite sure about and idea or principle, so you asked on here and had a chat about it, and it solidified into an issue you can create on BioJulia/Contributing