Who can I contact to discuss adding features to Apple's operating systems?
If you are suggesting new features or enhancements, try:

http://www.apple.com/feedback/

If you are reporting bugs, try:

https://bugreport.apple.com/

If you need to discuss extensive changes or a new direction you believe Apple should take, I suspect you will have to apply for a job with the department you believe your ideas will have most value in, and then find those who make the design decisions, and convince them that your ideas are the best direction going forward.