Good points: I thought I would create most of the level data client side and only replicate doors, containers and mobs. I was planning to spawn and de-spawn mobs dynamically based on player locations, but I do not know how this would play with everything else yet.
I’m keen not to re-invent anything that I can use already, but I don’t know enough to be sure what will work 
Can I create a sublevel dynamically in code and connect this to the level streamer? Edit: I realise that sounds weird… I mean I would dynamically create many sub-levels at game start and then stream them later.