This feature request can be summarized by the following three user sub-features:

1. Create more decoupling between certificate and issuer - a certificate should not need to specify issuer specific attribute (it happens especially with the AMCE provided certificates)

2. Allow for a default issuer (this probably makes sense only when cluster-scoped). This would allow third parties to package their applications to use cert-manager without having to know in advance which issuer they will need to use.

3. Allow for external contributors to add their issuer, without having it to be compiled in the cert manager's code.

/kind feature