Question(s) - sorry if they have been asked.
1. Do the zombies difficulty scale? so on day 1-21 with players level 1-30 the zombies are easier? lower XP and lower HP?
2. if, how hard would it be to add a scaling feature based on the games gate system?
Bonus if you could make a new NPC/Trader that I can drop on a server that would rock. in RUST I had a trader walking a path back and forth and if you tried to kill him he would one-shot you. he would sell stuff.
Good questions.
1. For regular days, the Snufkin Zombies do not use a difficulty scaling system. Their XP and HP remain the same.
2. The default game has a scaling system whereby days/games stages do not change XP and HP but introduce more feral, radiated, probability chance, and new types of zombies. The Blood Moon for the Snufkin Zombies also has this feature to some extent in that it introduces new zombies and also increasing their probability to spawn.
Based on the default system of introducing new zombies with scaling, here is an example of 'SnowZombies':
<entitygroup name="SnowZombies"><entity name="zombieSnow" prob="1"/></entitygroup>
<entitygroup name="zombieSnowGroupGS1"><entity name="zombieSnow" prob="1"/><entity name="zombieSnowFeral" prob="0"/></entitygroup>
<entitygroup name="zombieSnowGroupGS50"><entity name="zombieSnow" prob="0.85"/><entity name="zombieSnowFeral" prob="0.1"/></entitygroup>
<entitygroup name="zombieSnowGroupGS100"><entity name="zombieSnow" prob="0.7"/><entity name="zombieSnowFeral" prob="0.2"/></entitygroup>
<entitygroup name="zombieSnowGroupGS200"><entity name="zombieSnow" prob="0.55"/><entity name="zombieSnowFeral" prob="0.3"/></entitygroup>
<entitygroup name="zombieSnowGroupGS400"><entity name="zombieSnow" prob="0.4"/><entity name="zombieSnowFeral" prob="0.4"/></entitygroup>
<entitygroup name="zombieSnowGroupGS800"><entity name="zombieSnow" prob="0.25"/><entity name="zombieSnowFeral" prob="0.5"/></entitygroup>
This can be found in the default 7 Days To Die\Data\Config folder in the entitygroups.xml file. The format used in this scenario is similar in other groups and is typically tagged in other files as well.
1. The group 'SnowZombies' has been constructed to use a scaling system based on player or party game stage totals.
2. prob="1" tells us they will spawn with 100% certainty.
3. In the first instance after the SnowZombies entity group has been initiated, the sub groups are displayed for spawning.
For a very new player at game stage 1, zombieSnowGroupGS1 will spawn a 'zombieSnow' with 100% certainly but NOT spawn a 'zombieSnowFeral' with the same 100% certainty. No ferals at this stage.
However after getting some levels and attaining game stage 100, or maybe teaming up in a party and reaching game stage 100, zombieSnowGroupGS100 will become active. There is a 70% chance of a 'zombieSnow' and 20% chance of a 'zombieSnowFeral'. A slight increase in challenge weighted to increases player capability.
This can become further 'complicated' in the spawning.xml file whereby groups can be set for day/night/total alive at any one time/spawn delay, etc.
Based on this, it could take some time to append the existing files or create unique ones linked to game stage.
However, it can get as sophisticated as calling in a horde of a select group of only Snufkin's Zombies with or without default zombies the screamers (scout?) shrill cries bring in her buddies. The easiest way to implement such a feature would just be to add selective Snufkin Zombies to existing Game Stage groups; e.g.
entitygroups.xml
FROM:
<entitygroup name="scoutHordeStageGS8"><entity name="zombieBusinessMan"/><entity name="zombieSteve"/><entity name="zombieYo"/><entity name="zombieJoe"/><entity name="zombieMoe"/><entity name="zombieBoe"/><entity name="zombieNurse"/><entity name="zombieCheerleader"/><entity name="zombieSkateboarder"/><entity name="zombieArlene"/><entity name="zombieMarlene"/></entitygroup>
TO:
<entitygroup name="scoutHordeStageGS8"><entity name="zombieBusinessMan"/><entity name="zombieSteve"/><entity name="zombieYo"/><entity name="zombieJoe"/><entity name="zombieMoe"/><entity name="zombieBoe"/><entity name="zombieNurse"/><entity name="zombieCheerleader"/><entity name="zombieSkateboarder"/><entity name="zombieArlene"/><entity name="zombieMarlene"/>
<entity name="zombieScarecrow"/></entitygroup>
This can be done via a Mod which could add the Scarecrow zombie to the list, or manually via the game default files. This is Stage 8 and could be weighted with more challenging Snufkin zombies in combination as game stage increases.
In summary, the feature to change HP or XP does not currently appear to be implemented for zombies and balance was brought through modifying probability and spawn dynamics instead. Saying that, there is a scaling feature for the amount of XP received when looting so the code for such a feature exists.
It would be nice if modders more experienced in this domain could further expand where possible as this was a good question I'm sure the devs toyed with before reaching the current system.