I really have little idea just how much more dynamic levels are in Source 2, but seeing as we’ve moved from brushes to polygons, the possibility of freely removing and creating them seems worth asking for. I’d love for it to at least be looked into and to have some underlying engine features exposed.
Editing levels quickly without ever leaving the game would already be a good improvement, but being able to do so in multiplayer would allow an unprecedented amount of feedback and rapid iteration. You could be fixing errors or expanding options as a round is actively happening. You could develop maps extremely quickly with some helping hands.
When it comes to baked lighting, there’s simply no way around a proper result taking some time to finish. However, baking could be suspended until the user decides it’s a good time. The edited geometry could have cheap/dynamic lighting applied to it while waiting for a bake. There could be quick bakes and full map re-bakes. Changes don’t have to be seen by non-editors until published.
There was great progress made towards this idea with the old UE4 version, and it was probably the thing I was most excited for. The new Hammer’s really good and would still have its place, but it still requires tabbing out and fully compiling each time to my knowledge, which can interrupt the flow and is still a solo experience.
Alternatively, you could edit the new Hammer to support networking.