actually, a5e specifies that reactions can happen "at any time", presumably including on your own turn. i couldn't find a definition much more specific then that.
Ugh. Yep, that's RAW. I declare stupid.
What is the character reacting to on their turn??
I think you'd end up borrowing some of 3e's old problems. It's not quite so bad as full attack, but combat does get less dynamic if characters have to stick in place once they start swinging. In particular, you'd run into wasted actions if your fighter takes a target down and doesn't have anyone else in reach.
I want to waste fighter actions, at least when it comes to SA. That's the reason I'm suggesting that.
I have more of a problem with the six second round. Bob World Builder, after practice, could barely finish drinking a four ounce potion in six seconds. He had no time to move, do a bonus action, or reaction in that time. I know that combat is probably faster in real life but I'm fine with slowing it down, and taking longer, for the drama and story telling aspect of it.
This change is meant to pretty much remove save vs suck effects and eventually turn saves into a dynamic recovery capability.
I do like that 4E introduced something like this. It does mean having conditions build on each other. For example, turn to stone becomes slowed -> restrained -> petrified. It might require needing more conditions to do this but that works for me.
As for doing this with Stun or other similar conditions, I like the idea that once a creature has been stunned, they are immune for some amount of time, which is common with other abilities. Further, I think the stunned character should get another save at the start of their turn to have a turn and then it ends at the end of their turn. That also seems to follow other patterns.
Thanks for the discussion!