Well, for knockback you also certainly want to exactly specify how it is supposed to work.
In Mobs Redo, there is already a knockback, but only for the mobs and it is a very stupid form of knockback.
When you hit a mob, it moves backwards at a constant speed until it suddenly stops. This is so lame if you ask me, but the best approximation whe have now. I also don't want to blame TenPlus1 for this, because mod-created knockback is doomed to be bad anyways, because of lag, etc.
Also, knockback never affects players even in Mobs Redo because you can't set the velocity of players (yet). This sucks.
A knockback should definitely not be like in Mobs Redo but has to be more sophisticated.
Knockback also needs be tweakable by the subgame. I would hate to see hardcoded knockback values. Something like groups have been suggested before, and I kinda want to agree. But these ideas go not far enough.
Important knockback-related values which need to be considered:
1) How much knockback (force) a tool/weapon applies
2) How long it lasts
3) How resistant an entity is against knockback (should also allow for complete knockback immunity)
All 3 should be tweakable on a per-item and per-entity basis to be interesting. There could be weapons with weak knockback, strong knockback or no knockback at all. There could be enemies which are sometimes more, sometimes less affected by knockback, or they could become even immune to knockback. This opens many interesting possibilities.
Then, the question how a successfully applied knockback “wears off”. It clearly must not be like in Mobs Redo, where you have a constant speed and then it suddenly stops. No offense, but this is terrible.
Instead, the knockback should start at a maximum velocity and then smoothly (!) get weaker over time, until the velocity reaches 0 at which the knockback “is over”.