Use specific return values (one data class, or a sealed class) and a function for each request w/ all params you need for that request (you can also use retrofit interface and it'll implement them for you...), transform the response into that return value class so that your business logic doesn't have to understand your api contract but rather more businesss logic terminology.
Use dependency injection (like #dagger , #kodein or #koin) for mock or real dependency resolution, don't hard code them in.
You can look into #coroutines or #rx to avoid async tasks for the requests...
Yhere's probably lots of projects on github that could guide you... good luck!