Single Player reaction time is something that has been talked a while and quite a few people want, it would help diversify the way that you can play SP while also setting challenges for yourself. It would help a lot with MMA mod learning too as a big factor of slow progression in MMA is the fast reaction time where people just aren't used to it yet.

It would be relatively "easy" to add considering that you already have the base code set out for it,

IMO you could also set two modes for it too where one mode is normal reactiontime (such as the one we have in MP) where when the timer runs out both tori's are forced to do a turn.
The other mode would be sequential reaction time where you do a move for tori first then it moves onto uke after you press space or run out of time.

Not sure why there are no replies on this but I have been thinking about this and I realized that It could be very useful for practicing alone, want it to bump

This can and has already been done with a Lua script, there are no plans to have it added as a built-in feature.

You can find the script in this thread.https://forum.toribash.com/showthread.php?t=660812
(the "sprt" one)

While I can understand not building what was already implemented by the community I also believe this to be quite possibly one of the most beginner friendly ideas in toribash, I'd even like to see it implemented in the tutorial in someway. Being able to tell when your turn will end in game and also knowing how long you have to build your turn allows for new players to get to the experimental phase faster