Is the help action (to distract an ennemy) guaranteed in DnD ? If yes, isn't that a bit too easy to have an advantage ?
The help action is exactly what I had in mind. I thought it was a good idea to add it as a weapon action considering that it is more or less something Larian did.
The Help action (per 5e rules) is guaranteed. It's not overpowered because it costs an action to use. In almost all cases, it's better to use your action to attack/cast a spell. 2 attacks can hit twice, but a single attack (at advantage) can only hit once. I've seen a player character use the Help action in combat maybe 5-10 times in the entirety of my D&D experience, mainly involving grapples against single enemies.
I disagree about bonus actions being a punishment for some classes.
I know DnD a lot less than you do so I may be totally wrong about how "balance" everything is, but I like that Larian tried to add more possibilities to use the bonus action ("tried" because in my opinion they totally failed with shove, dip, hide, pommel strike and so on). I like that everyone have more choices to use bonus actions, especially classes that don't have some.
So I'm asking myself : Is the ranger class unbalanced / less appealing if he has to choose between hunters mark OR a weapon action ? I may not think about everything but I don't have the feeling that it punishes anyone, especially if weapons attacks are correctly balanced.
Adding more options isn't necessarily a bad thing; it is a common complaint that simpler classes like Fighter have nothing to do on their turns except move and attack. And true, you're not making the Ranger/Rogue
worse by adding these options.
If you're going to continue balancing things via Bonus Action cost, I suppose my advice is: make sure to keep in mind general bonus action availability. Standard Fighters don't get many options for their bonus action, so if you make a Special Weapon Attack more powerful than a normal attack due to it having an additional BA cost, then a fighter will ~almost always use that ability because why not? (E.g, BA Shove)
Best of luck! I look forward to this mod!