One relatively simple thing I'd love is the ability to re-run a job where it fails, which is something other CI systems have. On a current project we have some flaky Cypress tests and it sucks to have to re-run the entire workflow from the start when one of the last steps of the job fails. I'm definitely not the only one wanting this [1].
They also offer triggering workflows with 'workflow_run' based on other workflows, but that only happens on the default main branch. We auto build testing environments on each PR and I'd love to be able to have better workflow management based on branches.
[1] https://github.com/actions/runner/issues/432