There are quite a few encouraging signs that some form of Invasion will be coming to UT4. Reviewing GitHub commits for the Unreal Tournament project reveals that Epic’s Mysterial has several commits with references to Invasion, PvE, and monsters. Somewhat confusingly, there are also references to “Flag Invasion” which may be something entirely different.
The most likely scenario is that we will see some form of Invasion that will initially be available under the Custom tab at some point using existing player models as monsters.
The ultimate goal of course will be to bring alive some of the assets made in KazeoHin’s “Unreal Universe Monsters! Let’s make some art together!” forum thread and bring Invasion in as a standard part of the game. This will require a significant amount of work to get those assets properly functioning and animated in the game but it will be well worth it.