1) Remove field restrictions (very artificial) and let teams put fielders wherever they want, whenever they want.
2) Use the same ball for entire innings (use an extra layer of paint on the ball to avoid discoloration).
3) Reduce bat thickness. There is more wood in modern bats but at the same time, lighter, meaning the bat is generating more of the power in the shots than it was previously. Signficantly more power.
4) Remove the boundary rope. Use advertsing boards made with soft foam type material and set these as the boundary (positioned all the way back). This will give fielding team greater chance to catch bastmen out on the boundary.
These measures will create a more balanced game between bat and ball, but we should also be mindful that development of T20s has changed the mindset of the limited over batsman a great deal, so while the measures will help create a fairer game, I don't think they will bring the scores down to levels previously. Instead of threatening 400 plus scores regularly, I think we will get more 300-320 scores, but still not the 250. And to be honest, I think that is a good thing. The whole point of limited overs cricket is for people to be entertained. If you have too many games where the ball dominates the bat and we get scores of 230-250, it will lead to a loss of appeal on the part of the average fan.
Above all, people wnat to see batsmen scoring runs (not the bats), people want to see genuine sixes (not 50-60 metre ones), people wnat to see good bowlers have a say and not merely become fodder for batsmen. The pitches should offer something for both the bat and the ball.
We could also allow two bowlers 15 overs each. Too often, teams go in with a weak fifth bowler because they don't want to weaken the batting, which in turn usually leads to others making up the overs. And these contrived overs release pressure off the batting teams (even if they don't always cost huge runs).They can keep wickets in hand for the last 10-12 overs.