If I was to make the Monk stronger (which I don't think needs to be done, but still), then I would do the following, in this exact order.
1. (Only use this in a game that has short rest problems) Give Monks 3x the ki, and make ki recharge on long rests. As a possible variant, give 4x ki.
2. Provide Monks with the Mobile feat free of charge as 4th level bonus. This caters to the monk's strengths (more speed, free disengage) without being too powerful.
3. If the above two aren't enough, remove MAD dependency. Base monk chassis has two areas for wisdom; in unarmored defense and ki saves. Change the defense to 11+Dex Mod+ Proficiency and the Ki save to dex-based. If you want, create an alternate monk that is entirely wisdom based.
Those changes are more than enough, in my opinion, to make the monk almost OP.
Things I would NOT do:
A. Change the hd of the Monk. The monk is not a front-line fighter. It is not a d10 or d12 class.
B. Change the martial arts die.
1. (Only use this in a game that has short rest problems) Give Monks 3x the ki, and make ki recharge on long rests. As a possible variant, give 4x ki.
2. Provide Monks with the Mobile feat free of charge as 4th level bonus. This caters to the monk's strengths (more speed, free disengage) without being too powerful.
3. If the above two aren't enough, remove MAD dependency. Base monk chassis has two areas for wisdom; in unarmored defense and ki saves. Change the defense to 11+Dex Mod+ Proficiency and the Ki save to dex-based. If you want, create an alternate monk that is entirely wisdom based.
Those changes are more than enough, in my opinion, to make the monk almost OP.
Things I would NOT do:
A. Change the hd of the Monk. The monk is not a front-line fighter. It is not a d10 or d12 class.
B. Change the martial arts die.