TL;DR
There is a lot that could be improved with the replay system; surely it wouldn't hurt to take a look at it again... Here are a few ideas I came up with:

What ?
Freecam
Should be easy enough. There is already a dropdown for which player to follow, so why not have one for no player at all ...

I built a bus and upgraded one of the belts to red. Later I realised that the belts after the splitter were overly compressed. When iron is pulled from the belts later on, the compression does not shrink. When I redirect the belt just after the compression, the compression shrinks and disappears. I ...

I took 4 Spidertrons out for a stroll on automatic firing (3 of them were following the main one which I was driving). There were some biters and the Spidertrons fired explosive rockets at them. The biters were inside dense forest and therefore when the explosive rockets landed, the trees were ...