I am using this set of class libraries in three projects now. Obeying the Rule of Three I am generalizing most of it now. You should be able to reference these libraries from .net4.6, .net core 1.0, Xamarin and UWP. The more abstract the library the lower the required .NET Standard gets.

Abstraction	.Net Standard	NuGet
Backend.Fx	1.3

Environment	.Net Standard	NuGet
Backend.Fx.AspNetCore	2.0
Backend.Fx.AspNetCore.Mvc	2.0
Backend.Fx.NetCore	1.3

You get vendor locked to a set of abstractions, like my DDD building blocks and some architecture patterns defined as interfaces

Vendor	Library	.NET Standard	NuGet
Entity Framework Core 2.1 as persistence mechanism	Backend.Fx.EfCorePersistence	2.0
InMemory Persistence implementation	Backend.Fx.InMemoryPersistence	1.3
NLog logging	Backend.Fx.NLogLogging	1.6
RabbitMq Message Bus	Backend.Fx.RabbitMq	1.5
Simple Injector as DI container	Backend.Fx.SimpleInjectorDependencyInjection	1.3