I think the combination of speed control and camera freedom are amazing and can make every single creation EPIC, but some things could use more improvement, my suggestions might seem very specific but I'd love it if they were added:

1-I suggest making it possible to change the simulation speed using something like click and hold LMB and move it UP and DOWN or something? cause using that slider while the simulation is running is a pain for me.
Also, It'd be awesome to have the option to change the default controls(for wheels, cogs, hinges etc..) so I don't have to change it to WASD or something everytime I use a mechanical block.

2-As I said earlier, I think making cool scenes with camera angles and slow motion is one of my favorite aspects of this game, I think it would be AMAZING to have :
  +The ability to save cameras and switch between them, for example, being able to have a camera that is pointing towards the machine and one at a building, and being         able to switch between them using the "1","2","3" buttons?
  +To have a camera block that you can attach to your machine in build mode and can view it's perspective in simulation mode (would work nicely with the switching cameras    ability above^)

3-Last but not least, saving replays and sharing them etc...I don't know how difficult it would be but that'd be nice.

I love this game, I really hope the updates keep it fresh and exciting!

Thanks.