documentclass{article}
% RN. Sunday 17 December 2017
%=======================
usepackage[check-declarations]{expl3}
usepackage{xparse}
usepackage{tikz}
%-----------------------
ExplSyntaxOn
cs_new_protected:Nn __rn_start_tikzpicture:n
  {begin{tikzpicture}[#1]}
cs_generate_variant:Nn __rn_start_tikzpicture:n { V }

tl_new:N l_rn_auxOne_tl
tl_new:N l_rn_auxTwo_tl
tl_new:N l_rn_auxThree_tl

tl_new:N g_rn_tikzStyle_FR_tl
cs_new:Npn rn_tikzStyle_FR:n #1
%  PARAMETERS #1: dummy variable
%  RETURNS g_rn_tikzStyle_FR_tl for plugging into the tikzpicture     environment
  {
group_begin:
    tl_gset:Nx g_rn_tikzStyle_FR_tl {level~distance=15mm,
    every~node/.style={rectangle,draw,inner~sep=2pt,align=left},
    level 1/.style={sibling~distance=50mm},
    level 2/.style={sibling~distance=25mm}}
group_end:
  }  %  rn_tikzStyle_FR:n

tl_new:N g_rn_tikzContent_FR_tl
cs_new:Npn rn_tikzContent_FR:n #1
    %  RETURNS g_rn_tikzContent_FR_tl for plugging into the tikzpicture environment
  {
group_begin:
    % the nodes are visited post-order to generate tikzContent:
    % visiting node key = 6:
    tl_gput_right:Nx g_rn_tikzContent_FR_tl {child[missing]}
    tl_set:Nx l_rn_auxOne_tl {6}
    tl_set:Nx l_rn_auxTwo_tl {child{node{l_rn_auxOne_tl}}}
    tl_gput_right:NV g_rn_tikzContent_FR_tl {l_rn_auxTwo_tl}
    % visiting node key = 4:
    tl_set:Nx l_rn_auxOne_tl {4}
% ERR 1:    tl_set:Nx l_rn_auxOne_tl {4:$x_3^2$}
    tl_set:Nx l_rn_auxTwo_tl {node{l_rn_auxOne_tl}}
    tl_gput_left:NV g_rn_tikzContent_FR_tl {l_rn_auxTwo_tl}
    tl_set:Nx l_rn_auxThree_tl {g_rn_tikzContent_FR_tl}
    tl_gset:Nx g_rn_tikzContent_FR_tl {child{l_rn_auxThree_tl}}
    % visiting node key = 10:
    tl_set:Nx l_rn_auxOne_tl {10}
% ERR 2:    tl_set:Nx l_rn_auxOne_tl {color{red}10}
%    tl_set:Nx l_rn_auxTwo_tl {child{node{l_rn_auxOne_tl}}}
    tl_set:Nx l_rn_auxTwo_tl {child{node[circle,draw,inner~sep=2pt]   {l_rn_auxOne_tl}}}  % this works well!
    tl_gput_right:NV g_rn_tikzContent_FR_tl l_rn_auxTwo_tl
    % visiting root, key = 8:
    tl_set:Nx l_rn_auxOne_tl {8}
% ERR 3   tl_set:Nx l_rn_auxOne_tl {8\BF:+1}
% ERR 4   tl_set:Nx l_rn_auxOne_tl {textbf{8}}
    tl_set:Nx l_rn_auxTwo_tl {exp_not:N node{l_rn_auxOne_tl}}
    tl_gput_left:NV g_rn_tikzContent_FR_tl l_rn_auxTwo_tl
group_end:
  }  %  rn_tikzContent_FR:n

cs_new:Npn rnTikZ_showTree:n #1
  {
group_begin:
  rn_tikzContent_FR:n {#1}
  rn_tikzStyle_FR:n {2}
  __rn_start_tikzpicture:V g_rn_tikzStyle_FR_tl
    g_rn_tikzContent_FR_tl ;
  end{tikzpicture}
group_end:
  }  %  rnTikZ_showTree:n

NewDocumentCommandmyShowTree{O{5,3,4,7}}
  {
    rnTikZ_showTree:n {#1}
  }

ExplSyntaxOff
begin{document}
section{Native tikzpicture: Basic}
begin{tikzpicture}
[level distance=15mm,
every node/.style={rectangle,draw,inner sep=2pt,align=left},
level 1/.style={sibling distance=50mm},
level 2/.style={sibling distance=25mm}]
node{8}child{node{4}child[missing]child{node{6}}}child{node{10}};
end{tikzpicture}

section{Native tikzpicture: Enhanced node content}
begin{tikzpicture}
[level distance=15mm,
every node/.style={rectangle,draw,inner sep=2pt,align=left},
level 1/.style={sibling distance=50mm},
level 2/.style={sibling distance=25mm}]
node{textbf{8}\BF:+1\H:2\W:4}child{node{4\$x_3^2$}child[missing]child {node{6\$alpha_2$}}}child{node[circle,draw,inner sep=2pt]{color{red}10}};
end{tikzpicture}

section{`tikzpicture` environment generated}
myShowTree[8,4,6,10]

end{document}