I’m curious if there will be Dynamic NavMesh available?
I’m spawning a lot of objects during in-game and it would be nice if AI can avoid them.
What kind of objects are you spawning? Unless you are building a labyrinth you probably don’t need to update the NavMesh. The NavMesh is not used to avoid objects but rather to find a path from point A to point B on the map. Avoidance is then handled more “locally” when required. I don’t know what tools are available for this in UE4 but regardless Boids is a good place to start.
I’m spawning Pawns and Actors - they are going near player and they are colliding with each other. I’ve enabled Update NavMesh in Runtime but it isn’t working for Characters but it’s working for projectiles…I can see that navmesh is rebuilding when I’m shooting 
Figured this out.
In Project Settings → Navigation System you need to enable Build Navigation at Runtime.
If you want to use UE4 avoid system just enable Use RVOAvoidance and set Weight to 0.5 in CharacterMovement component.