Just had this thought, and figured surely someone has already written on this. But if they have, Google doesn't appear to have figured out where they posted it.
Maybe it's too simple.
You know those football helmets that QBs and one player on defense get... the ones with a small lightweight speaker inside them?
Pitcher and catcher each get earpieces.
No more visits from the dugout to the mound. No need, so non-existent. To be clear, all that time wasted by pitching coach or manager walks to the mound are no more--they can talk to them directly. "How we want to pitch this guy" is relayed directly, just like the QB is told what play is to be run. Pitching change? No more waiting. The guy on the mound gets told he's being taken out, and he just starts walking to the dugout while the bullpen guy come in.
Mind you, sometimes you just want your catcher to go out and kill some time to give your pitcher a breather or your infield wants to huddle on how to play a situation; and there's nothing to prevent those huddles. They just don't involve waiting for someone to walk from the dugout or back to the dugout in order for them to occur.
Here's what else will save time. No more waiting for the catcher to give a sign. Better, with a runner on second, no more waiting for the catcher to give a sign, then give another, then give another, then decide to re-start the cycle all over again because the battery can't seem to get it together.
Rather, the call is made from the dugout by the pitching coach or manager, ie, whichever has the mic... and though the pitcher could still decide to shake off the first call, and succeeding calls until he gets what he wants to pitch... it's still naturally going to shave off seconds from every batter's plate appearance.
All told, this one change reduces seconds and even minutes from the first pitch through the last out.
All we need is a minor league to introduce it and pilot test it.