Most AMD Threadripper board partners have already shown their X399 motherboards a bit to the public, MSI however has not. MSI presented X399 GAMING PRO CARBON AC motherboard.
This is a high-end motherboard with swappable heatsink covers, 13-phase power design, three M.2 slots and four reinforced PCIe slots. Below some slides that will give you an overview of what to expect, slides courtesy of videocardz: