I can think of a few:
1. Stop covering pitches in between matches (going back to uncovered pitches ought to make Geoff Boycott happy)
2. To be eligible for Test matches, pitches have to be prepared in a standard way that allows for a fair contest between bat & ball.
3. Each team has 2 innings of 90 overs max. Should stop teams dragging out innings when batting and reduce the problem of slow over rates when bowling. Should also reduce likelihood of draws.
4. Batting powerplays / fielding restrictions during overs 30-40 and 80-90 (yeah I nicked this from ODIs). Increase scoring potential despite limited overs.