Ok one way to achieve it (kind of) is to make the Collectibles, Powers…very generic and with a lot of properties exposed. By doing this, a modder/designer can simply extend a blueprint form it and tweak the properties to create something entirely new. For instance in a hoby project that I am working on, I have powers that I caan assing to charcters. Different powers do diferent tings, some increas/decrease health, some affect speed, resistance… So initially I created sveral powers and hardcoded their efefcts. Later I created a base class Power and an interface which defined some functions like (ApplyEffects, GetHealthBonus, GetReistanceBonus…). When I want a new power, I extend th Power class and tweak the values for variables like HealthBonus, ArmurBonus, InstantBonus, OverTimeBonus, BonusDuration, CastEffect, HitEffect, …(actually there was well over 50 proeprties). By doign this I was able to implement a projectile based power, self healing power, …all using the same class. One problem with this approach is you fisrt need to identiofy all the gameplay specifics and how they all work toegthor. If you intend to change gameplay mechanics sometimes later, then this setup will bite you.
About Blueprints and Git. Yes binary files are a problem with version control unless the version control system know how to parse these files and identify the differnces (like the Bleupritn Diff tool). I belieave unless EPIC adds a plugin for Git version control, we have to make sure only 1 person works on any file at a time …though it kind of defeats the purpose of Git.