In this case, I would use to call an web API which could be based on microservices architecture for contacting different LoB systems - including SP Online, on-premises server, etc.
This architecture could give you the benefit of running different DLLs (required for both online and on-premises - they use different DLLs) in their own context (as a daemon). For example, in our implementations we call these "adapters" and we just plug them into our "service bus" application, expose adapter properties (for example these could be credentials or query parameters) through an interface and we work with any LoB system that way.
