Setting a default library when uploading media content in a related media field is not possible.
A feature request can be logged through the Sitefinity Ideas Portal. Customer feedback is valuable and feature requests are monitored by our Product Management team. Feature requests are reviewed during the planning phase of each new product release. Once a feature request is submitted the Sitefinity Community will have the opportunity to comment on and vote for the feature request.
For detailed information on how to submit a Feature request, please refer to Knowledge Base article 000074190, How to submit a Sitefinity feature request