Networking in C++ and more specifically server authoritative movement, client prediction, and especially the FSavedMove side of things. You guys are killing it with Fortnite but there is still no cohesive C++ networking course that goes farther than BP (which is mirrored by many youtubers) and a handful of partially finished or very basic C++ videos.
I recently had to shelve a VR multiplayer game because there is next to no documentation on how to setup or use any of this and the server corrections over 50ms of difference were enough to get me sick and render the game unplayable. I have all the Udemy courses but even following the one by Ben Tristram was a catastrophic failure using a somewhat custom movement system (notably jetpacks). They supposedly replaced the server/client correction system with one that was intended to have the server behind the client with packet timestamps but as soon as I used it in my project it did not work at all. The only thing I have to go off of is a wiki post from UE 4.14 and reading the Unreal Tournament source code but that is going into a grey area because I cannot use UT code in my projects.