If you want to do a retake of 4e, you should start with the character builder (and maybe monster builder).

Compatibility can mean a few things.

0. The playstyle is *like* 4es, on the DM's side.

1. The playstyle is *like* 4es, on the player's side.

2. You can rebuild 4e archtypes in this system with similar feel.

3. You can use existing 4e monsters and encounters drop-in. (3b: with some modifications)

4. You can use existing 4e characters drop-in. (3b: with some modifications)

5. You can mix and match 4e feats and options with your heartbreaker system characters and feats (4b: as an option that isn't as balanced)

(order is not strict)

Pathfinder aimed for 5b. I'd aim for 2, 3 or 3b, or maybe even 0.

But before considering it, you need a character builder. I would probably write it as a mobile app.

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My personal Heartbreaker would steal things from 5e.

A character starts with a level in a Race, Background, Class and Subclass. Each grants a HitDie and corresponding HP.

I'd use presumed competence (attributes don't effect accuracy of abilities, rather effect size) and bounded accuracy.

Proficiency bonus scales logarithmically. It goes up at 1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, 64, etc.

So you start with +4 proficiency. At L 20 in class and subclass you have 42 HD, so a +10 proficiency bonus (and are roughly 10x as powerful as a starting character in damage and HP).

Proficiency boosts both attack and defence. Unlike 5e, "Saves" are univerally boosted by Proficiency. "Good saves" will be represented by something like a static bonus or advantage.

Balance wise, I'd want encounter building to be as easy or easier than 4e. One thing I'm trying to get to work is to make encounter balance be "add up HD". This seems to require that numbers add a significant advantage over single big foes (an outnumber mechanic?) to make up for proficiency gap and the ability to attrition weenies down to reduce damage flux.