TMyClass = class
  public
    function GetRandomClass<T: TBaseClass>: T;
  end;

var
  C: TMyClass;
begin
  C.GetRandomClass<TFirstClass>().MethodFirst
  C.GetRandomClass<TSecondClass>().MethodSecond

TMyClass = class
  public
    function GetRandomClass<T: TBaseClass>(const aIndex: Integer = 0): T;
  end;

var
  C: TMyClass;
begin
  C.GetRandomClass<TFirstClass>(0).MethodFirst
  C.GetRandomClass<TFirstClass>(1).MethodFirst
  C.GetRandomClass<TFirstClass>(2).MethodFirst
  C.GetRandomClass<TSecondClass>().MethodSecond

  TA = class
  end;

  T1 = class(TA)
  end;

  T2 = class(TA)
  end;

  TMy = class
  private
    type
      TWrapper = record
      private
        FObject: TValue;
      public
        constructor Create(aObject: TA);
        function AsObject<T>: T;
      end;
    function GetA(const Index: Integer): TWrapper;
  public
    property A1: TWrapper index 0 read GetA;
    property A2: TWrapper index 1 read GetA;
  end;

{ TMy.TWrapper }

function TMy.TWrapper.AsObject<T>: T;
begin
  Result := FObject.AsType<T>;
end;

constructor TMy.TWrapper.Create(aObject: TA);
begin
  FObject := aObject;
end;

{ TMy }

function TMy.GetA(const Index: Integer): TWrapper;
var
  FList: TList;
begin
  Result := TWrapper.Create(FList.Items[Index]);
end;

procedure TForm6.FormCreate(Sender: TObject);
begin
  TMy.Create().GetA(1).AsObject<T1>.First
end;