I think the biggest missing thing that I notice in new engines is that no one is seriously making a game with them. I know, making the engine is more than enough work already, but if it's not you having _somebody_ aiming to release a product made with the engine ought to be somewhere in your priorities. You might have lot of interesting tech, but unless end-user developers are using it in real ways you might find out later you're not headed in the right direction and meeting real needs. You can see this with the difference in emphasis between Unity (not game developers) and Unreal (Epic definitely are), to Unreal's benefit. And I say that as someone who uses Unity in my full-time job every day.
... but of course, you need a base feature-set to attract devs. So it's a big of a chicken-and-egg problem.