Thanks so much for taking on developing this based on CoreRPG. I'd keep giving you rep, but I need to spread it around first!
Layered rulesets are very similar to extensions - in terms of overriding ruleset code and config. The main differences between an extension and a ruleset are as follows:
1) A ruleset allows you to release DLC (modules and extensions) specifically tagged to be opened only in that ruleset. The ruleset can be set to also allowing opening of modules and extensions for other rulesets using the
importfrom tag.
2) Extensions are specific to ruleset/s and can be turned on and off without disrupting the base campaign data - so they're good for setting changes or options to a base ruleset. However, they don't provide a restriction of which extensions or modules can be opened.
You could go either way with this. If you think there could be modules that would be shared across Genesys and Star Wars then make the Star Wars code changes in an extension. This would also be the case if you think GMs might want to use an existing campaign they've played in for a while a shift it from Genesys to Star Wars or vice versa. If you think you'd like to keep them separate then go with a ruleset.
In the end, either decision is not a dead end - you can get around either restriction with the
importfrom setting or by manually editing the campaign ruleset tag.