Roboumps and strike zone modification would be my preferred as they don't really change the fundamental nature of the game.
If that just isn't working, look at moving back the mound.
I for one have loved the 3 batter rule. Pitching changes wreck the pace of the game.
None of that was anything new in the thread, so I'll take a stab at something.
What about eliminating warm up pitches when the BP guy comes in? (or greatly reducing) They have been throwing in the pen, why do they need to throw a bunch more from the mound? I've never pitched at any kind of competitive level, so maybe this is extremely important, but maybe not. Obviously not including injury switches.
What I like about this is that pitching changes become a very quick affair and don't break the pace or stop an intense moment.