Suggestion for a feature long missing from the TIM series
From back in the days of DOS and TIM1, one of the great issues I've always had is trajectory. There's a LOT of trial and error in aiming (I at one point stopped playing TIM2 and went over to playing LaserLIght purely because of it having a Hold Right Mouse to show 8 directions for aiming purposes feature).
Implementing an aiming feature might be too tricky and messy in the UI to keep track of (unless you were to do some sort of tracing certain objects on a blueprint view or similar after a run) Instead, how about a blue tinted (blueprinted?) snapshot/savepoint. Mark it during the run of a contraption, and state at that exact point, including speed and such, will be stored. You can then make changes to the layout and replay from that point a couple of times to get aim right. Of course to complete a puzzle you'd need it to be run from start.
Anything put in the way of something which happened previously wouldn't affect the snapshot. That's up to the player to keep track of. Making a collision layer for all the elapsed time would be too tricky and CPU and memory consuming I'd fear. Also placing dynamic objects would need to be wound up to the snapshot time once placed, maybe with a neat pencil sketch animation to show it's expected movement in relation to the snapshot?
A snapshot wouldn't eat too much memory unless you have huge contraptions, would be a fairly simple concept to convey to players, and would, at least for me, be very welcome in tweaking positions without having to replay a contraption from start to move a few pixels. Or lacking a snapshot, how about a "run as fast forward to XXsec" to mimic this behaviour without the extra code to store and manage a snapshot?