I don't know if this was brought up, but I was wondering if anyone has played with allowing certain activities to span turns. For instance, allowing a spell caster to move and then spend 3 actions to do a full-blast Magic Missile. This would have some drawbacks of course, such as allowing bad guys to run over and kick a casting caster. I'm thinking of allowing this in one of my games but I wanted to know if anyone else had any experience with this.
I would rule you couldn't spend *more* than 3 actions on a given activity (like MM for instance) and you can't ready an action that takes more actions than you have left (so move twice and ready a strike would be a no-go in this instance).
This would allow someone to spend the minute in combat to unlock the door while everyone else is protecting the rogue as they try to escape, also the aforementioned spell caster using 2 actions to cast one spell, another to start casting, and the first action on their next turn to finish that second spell (and allowing them to cast a third in 2 rounds...)
So what you're describing was how it worked in the Unchained Revised Action economy, and I played around with it some. It was OK, but IMO was largely unnecessary complexity. I allowed it for unchained, but I also allowed a lot of stuff that didn't survive the edition change, like unchained flurry of blows being one action for two attacks without penalty, or similar stuff with multishot and rapid shot.
Seeing as how those martial options are somewhat weaker, I wouldn't introduce something that can strengthen casting action economy that much into this version of the game.