Face ID needs to get faster and more accurate, which are two things that I fully expect Apple to improve in future versions of Face ID.
Apart from speed and accuracy, Face ID needs to register faces even when they're not specifically looking directly at the iPhone X. But herein lies a problem: Looking directly at the iPhone X to unlock it is a necessary security feature. Otherwise, anyone could pick up your iPhone X and point it in your face's general direction when you're not expressly looking at it.
So, with all that in mind, the ultimate solution is to bring back Touch ID! There's no telling whether Apple will re-introduce fingerprint sensors into the next iPhone X. For one, any fingerprint sensor would need to be hidden underneath the display so that an iPhone X successor can keep a bezel-less design. Secondly, hidden fingerprint sensors would need to be almost flawless.
The technology is here, as I saw it at this year's CES in smartphone from Chinese phone maker Vivo. But it's still in its early stages, and I'd understand if Apple is waiting for the technology to improve.