You can post them here, or on the user mailing-list or could naturally also post the enhancement request directly in https://bugzilla.proxmox.com/
You should add a real world use case and it should be a feature which is useful for more than a few specialized use cases.
We normally redirect then the ones we deem sensible over to https://bugzilla.proxmox.com/ (if not already created there), so that we can keep track of it.