Tikz fit inner sep seperate values for all 4 directions

You can add some points to fit around. For example, to get

use the code:

\documentclass[]{standalone}
\usepackage{tikz}
\usetikzlibrary{fit}

\begin{document}
  \begin{tikzpicture}[every label/.style={text=gray},node distance=2cm]
    \node[draw] (n1) {hello};
    \node[right of=n1,draw] (n2) {hello2};
    \coordinate[above left of=n1] (n1left);
    \node[draw,fit=(n1) (n1left) (n2)] (t31) {};
  \end{tikzpicture}
\end{document}

This comes with a key fit margins that does what I think you want. You can use it as e.g. in

\node[draw,fit margins={left=3pt,right=12pt,bottom=3pt,top=8pt},fit=(n1) (n2)] (t31) {};

to obtain

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{fit,positioning}
\tikzset{fit margins/.style={/tikz/afit/.cd,#1,
    /tikz/.cd,
    inner xsep=\pgfkeysvalueof{/tikz/afit/left}+\pgfkeysvalueof{/tikz/afit/right},
    inner ysep=\pgfkeysvalueof{/tikz/afit/top}+\pgfkeysvalueof{/tikz/afit/bottom},
    xshift=-\pgfkeysvalueof{/tikz/afit/left}+\pgfkeysvalueof{/tikz/afit/right},
    yshift=-\pgfkeysvalueof{/tikz/afit/bottom}+\pgfkeysvalueof{/tikz/afit/top}},
    afit/.cd,left/.initial=2pt,right/.initial=2pt,bottom/.initial=2pt,top/.initial=2pt}
\begin{document}
\begin{tikzpicture}[]
\node[draw] (n1) {hello};
\node[right=of n1,draw] (n2) {hello2};
\node[draw,fit margins={left=3pt,right=12pt,bottom=3pt,top=8pt},fit=(n1) (n2)] (t31) {};
\end{tikzpicture}
\end{document}

I also employ the positioning library for easier positioning.