\documentclass{article}
\usepackage{expl3,xtemplate,kantlipsum}

\ExplSyntaxOn
% contents to produce on the page
\cs_new_protected:Npn \__mytest_cover_title:
  { testcovertemplate }
\cs_new_protected:Npn \__mytest_cover_img:
  { \parbox {5cm} { \kant[42][1-3] } }

\tl_new:N   \l__mytest_content_tl
\tl_new:N   \l__mytest_begin_align_tl
\tl_new:N   \l__mytest_end_align_tl
\skip_new:N \l__mytest_top_skip

% creates an object type “nju” XD
\DeclareObjectType { nju } { \c_zero_int }

% defines several properties of a single page element
\DeclareTemplateInterface { nju } { page-element } { \c_zero_int }
  {
    align    : choice { l, r, c, n } = c,
    top-skip : skip = \c_zero_skip,
    content  : tokenlist
  }

\DeclareTemplateCode { nju } { page-element } { \c_zero_int }
  {
    align =
      {
        l =
          { \tl_set_eq:NN \l__mytest_begin_align_tl \flushleft
            \tl_set_eq:NN \l__mytest_end_align_tl   \endflushleft  },
        r =
          { \tl_set_eq:NN \l__mytest_begin_align_tl \flushright
            \tl_set_eq:NN \l__mytest_end_align_tl   \endflushright },
        c =
          { \tl_set_eq:NN \l__mytest_begin_align_tl \center
            \tl_set_eq:NN \l__mytest_end_align_tl   \endcenter     },
        n =
          { \tl_clear:N   \l__mytest_begin_align_tl
            \tl_clear:N   \l__mytest_end_align_tl                  }
      },
    top-skip = \l__mytest_top_skip,
    content  = \l__mytest_content_tl
  }
  {
    \AssignTemplateKeys
    \null \skip_vertical:N \l__mytest_top_skip
    \group_begin:
    \l__mytest_begin_align_tl
    \l__mytest_content_tl
    \l__mytest_end_align_tl
    \group_end:
  }

% the title example
\DeclareInstance { nju } { cover / title } { page-element }
  {
    align = r,
    top-skip = 5cm,
    content = \__mytest_cover_title:
  }

% the img example
\DeclareInstance { nju } { cover / img } { page-element }
  {
    content = \__mytest_cover_img:
  }

% user interface
\NewDocumentCommand \MakeTestCover { }
  {
    \thispagestyle { empty }
    \clist_map_inline:nn { title, img }
      { \UseInstance { nju } { cover / ##1 } }
  }
\ExplSyntaxOff

\begin{document}
\MakeTestCover
\end{document}