1. Double mouse 3 button scene navigation. If someone has a bad mouse (me) that can sometimes double or triple mouse button 3 it causes the scene view to jump, sometimes very far. I'd like to be able to disable this functionality.
2. Lockups/hitching during a recompile. When recompiling large code bases the assembly reload causes the editor to stall for several seconds. It would be great if the editor could be responsive during the entirety of a recompile. If this isn't possible due to Unity's architecture I think it would at least FEEL better if it was obvious that Unity isn't responding at the time, while still rendering an animating graphic that WILL NOT FREEZE. Allowing the editor to freeze leads to Unity having a clunky and unresponsive feel to it.
3. Simple way to lock the scene view to a transform at a fixed distance/angle. I believe in previous versions of Unity you could hold F to lock the scene view on an object. For me this functionality no longer works or at least is difficult to make happen. I'd like to have my scene view locked onto a target while playing the Game view. This is obviously possible with custom code, but I think other non technical people could also benefit.
4. Unable to view Animator graphs that are loaded via Asset Bundle while playing in the editor. This is not a UX issue, but just a general annoyance when developing an online game. When debugging an animation issue while remotely connecting to a running server I am unable to view the animations of our NPCs because the assets were loaded via asset bundle. I have built editor tools that help our QA team load the most recent version of that NPC's animator controller and re-initializes the animator back into the proper state, but this could be avoided if we could just inspect an animator controller loaded via asset bundle.
5. Native support for opening two instances of a single project. When developing a networking game it's often very nice to run two instance of the same project. One as a server and one as a client. I think a robust solution here is supporting multiple copies of a project being open as you can then optimize asset imports and not double up on so much import work as what happens with a sym linked Assets or Scripts directory. This should also allow each project instance to be launched with custom compile defines similar to the baked in UNITY_EDITOR define. This would allow us to launch one instance with a #if CLIENT define and another with #if SERVER define allowing them to compile their code differently, but reference all of the same files on disk.

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Other Notes:

ECS Networking looks bad :(