I suppose an interesting question is how to handle "elite" and "solo" monsters. A Dragon is usually a solitary enemy but a difficult challenge. SO naturally, it will have a lot of hit points. But is that alone sufficient, or doesn't a party actually get a certain advantage against him thanks to having multiple actions. Even if most abilities, even charms and dominates and the like, are based or limited by the target's hit points, some abilities will not. Isn't the Moradin's Defender ability giving an enemy disadvantage on his attacks? Imagine that used against a high hit point creature - it's far more powerful than it normally would be against a lower hit point monster with allies.
Or will high level creatures (excluding PCs?) by default gain extra actions?