Today, the vSphere Content Library only supports a single deployable VM type using the Open Virtualization Format (OVF) standard. Although customers are familiar with both OVF and OVA (archive of OVF and VMDKs), support for vCenter VM Template is still one of the most highly requested feature for Content Library. This should come as no surprise since many of our customers have built operational procedures and automation workflows for managing VM deployments over the years and simply switching to another format has a significant impact to their existing workflows.
UPDATE (10/18/18) - The VMTX feature is now available in vSphere 6.7 Update 1
I know the Content Library team has been heads down working on a number of enhancements to Content Library and it looks like one of these improvements has recently made its way out onto VMware Cloud on AWS (VMC) which I had just noticed while working in my SDDC.