I come at this from a bit of a different direction. The underlying premise of tinkering with the pace of play is, I think, to make the game more enjoyable to a wider base of fans. I get that, but I don't think it is the most obvious solution or the most pressing problem. If you want a good example of horrible pace of play, just look at the NFL, where they allot about 4 hours of TV time to cover a game that will have, at most, only 60 minutes of action and, in actuality, a lot less than that. They seem to be doing just fine drawing fans.
On pace of play, I like very much the ideas of a pitch clock and "stay in the box" for batters. I have no interest in robo umps or challenges on ball/strike calls. The umps have the technology to police themselves, assuming they are encouraged or pressured to do so. Challenges do little more than waste time. Maybe have an extra ump in the booth and if someone really screws the pooch on a call in the field, stop play, correct it, and move on. No wasted time on ball/strike calls, please!
In terms of making the game more interesting to a wider fan base, a couple ideas that I would enjoy hearing some views on:
1. Fire MLB's entire marketing department. It appears they don't have any idea how to effectively market the game, the teams, or the players.
2. Encourage players to have fun. I think this is improving, but there is still a fair amount of old-school retaliation for those who violate the "rules". Part of the answer could be to discourage the retaliation. Maybe a hit by pitch that results in a warning to the pitcher gets the batter two bases instead of just one.
3. Pick a weekend during the season and bring to the U.S. the top 30 international teams for a 3 game series with an MLB team. Rotate teams every year. Count the games as inter-league games and have them count in the standings.
4. Instead of rewarding the worst teams with the top draft picks, relegate one team from each league to the International or PCL leagues. Top two teams from those leagues join the majors the next year. This would probably require an end to "team control" over players and certainly an end to affiliation with MLB teams at the AAA level -- it wouldn't be fair to a top AAA team to have their stars called up to the mother ship during the course of a season, especially if they have a shot at moving up. Relegation would also require some tinkering with the draft and rules of player control and arbitration, as the AAA teams would need adequate access to the player pool. Maybe the answer is a free market in talent at the MLB and AAA levels, with no team control beyond the terms of each player contract.
That's it for now. If I think of any more crazy stuff I will chime back in.