The addition of a replay feature so you can watch the finished game. In first person (follow player) or spectate mode.
This will help content creators to make cool videos not only from a first person perspective but also see the same play from a different perspective or enemy view. A top view of the chaos from two teams meeting would be epic.
In addition this would help identification of possible cheaters and make better bug reports better because you can replay the part where the bug appeared or leave the time stamp in the bug report.