Multiple SSL for single Application with several domains (not subdomains)
Currently, you are only able to install one SSL certificate per application. However, with WordPress multisite installations, a single application could have several different domains. Each domain needs it's own SSL certificate.
Without this feature, we can't have large multisite installations running sites with different domains that need SSL.

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Dinesh Sharma commented
I second this...
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Phill Coxon commented
For WordPress multisite installations we need to have a wildcard SSL cert for the main domain and subdomains AND the ability to add additional let's encrypt (or paid) SSL certificates for each custom domain that is mapped to a subdomain. The additional let's encrypt SSL certificates need to be able to be added / removed via the API.