public IEnumerable<T> Ancestors<T>() where T : OpenXmlElement;
        public IEnumerable<OpenXmlElement> Ancestors();

    interface IInterface
    {
        IList<T> GetList<T>() where T : MyClass;
        IList<MyClass> GetList2();
    }

    public class MyClass
    {
        public string Name { get; set; }
    }
    public class MyClass2: MyClass
    {
        public string  Text { get; set; }
    }

    public class Worker: IInterface
    {
        public IList<T> GetList<T>() where T : MyClass
        {
            return new List<T>(new[] {  (T)Activator.CreateInstance(typeof(MyClass)), (T) Activator.CreateInstance(typeof(MyClass2)) });
        }

        public IList<MyClass> GetList2()
        {
            return new List<MyClass>(new[] { new MyClass(), new MyClass2()  });
        }
    }

 public IList<T> GetList<T>() where T : MyClass
        {
           .....
        }