I think you may create a Workflow Scheme to each project and to add a Post Functions (Update Issue Field) in the first transition of your workflow (Create), then you will set which Priority you want to use.
by default priority you mean that you would to have preselected priority field on create screen?
Update: Version 1.2.0 have just been released, where it's possible to indicate Default value for each Project & Issue Type pair.
please fell free to raise an issue for that on our service desk:
that's definitely something what we can add to Extended Schemes for JIRA not only for priorities but for Resolutions and Link Types too.
Jira may be a staple part of your workday, but your colleagues, customers, business partners or an auditor might need information presented in a different format, that is easy to access and read. And...
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