Proposals for GitLab, the product, are encouraged in the gitlab-org/gitlab project. You can use one of the issue templates, for example https://gitlab.com/gitlab-org/gitlab/-/issues/new?issuable_template=Feature%20Proposal%20-%20lean
You can create an account on GitLab.com SaaS, and create a new project in your namespace, and test away. If it helps, you can see plenty of playground projects in my profile. Michael Friedrich · GitLab which trigger no-one’s notifications.
I’d suggest starting a proposal issue, and explain the why and alternatives considered.