In baseball, a hitter should be allowed to bring the bat with him on the base path. As he circles the bases and the ball comes near, he should be allowed to bat it away in order to defend himself, as well as use the bat as a bayonet in order to discourage the defense from blocking bases.
The decision as to whether or not it's appropriate to bring the bat along should be at the runner's discretion. If he feels it's worth the potential benefits in exchange for the inherent slowdown, he should go for it.
Also, the sport should be co-ed. As backwards as car racing is, at least it has the wherewithal to respect gender equity enough to recognize that both sexes can compete on even ground in skill-based activities. This isn't arm-wrestling here, it's hand-eye coordination.
Tennis should also go co-ed, beyond the inconsequential gimmick of mixed doubles. Sure, there would probably almost never be female champions, but think of the great theater that would emerge if a woman ever came close. Golf, which should also go completely co-ed, has made strides in this area.
Furthermore, there should be no weight classes in boxing, MMA or any other sport. Creating little affirmative-action leagues for people of all shapes and sizes muddies things up and disrespects the little guys, assuming they could never compete with the bigger dudes so why not even try?
If weight classes stay, there should be an NFL for small, slow, weak, old guys.
As for the NFL, holding penalties should be done away with, just as blocking and charging in basketball should be tossed aside. Both fouls are subjective judgment calls that are based on the whims of officials, and both occur on almost every play, so just let the players fend for themselves and see how things work out.
Also, they should just stop with the NHL because nobody cares. Bring it out every four years for the Olympics a la curling.