Is there a "best" forum for suggesting improvements to CiviCRM?
I think it would be useful to ask a self-serving question in SE and then answer it yourself. I think SE has a more public audience and hence improvements get noticed.

In addition, I would suggest starting off on one of the new chat.civicrm.org channels depending on whether it is 'hey i have this new idea' (maybe town square) or 'i need to insert x in to y while not breaking z' perhaps in the developer chat.

hth