1. Remove OD, use hit error as the primary method for scoring plays
In my experience, hit error (visible in-game as 'unstable rate') is the most accurate measure of performance on a map.
Problems with Overall Difficulty (OD) / hit windows:
-The same play can give a different score if the OD is different.
-Because hits can only give one of four arbitrary values (300, 100, 50, 0), there is a significant loss of information.
Problems with Max Combo:
-A single miss near the beginning or end of a song will have much less impact on the score than one in the middle.
-There is a strong incentive to avoid longer maps and instead 'farm' short maps where the chance of making a play with no misses is higher.

2. Add features that encourage skillful play:
-Accuracy for slider heads (already in lazer)
-Accuracy for slider ends (?)
-Reduce the slider follow circle radius (Is currently 2.4x hit circle radius. My testing shows that 2x would be fine)
-Make unnecessary clicks count as misses

3. Remove HP Drain / Failure
In my opinion, there's no good reason to prevent the player from being able to play the whole map for failing to meet some arbitrary threshold of performance set by the mapper. Bad plays will be bad plays.

4. Abolish PP in favor of an Elo-type system based on leaderboard rankings
The way PP is calculated is convoluted and arbitrary. How 'difficult' a map is is subjective and difficult to quantify with an algorithm.