\documentclass{article}

\usepackage{filecontents,lipsum}

\begin{filecontents*}{MyPackage.sty}
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{MyPackage}
\RequirePackage{environ}
\newif\if@hidden% \@hiddenfalse
\DeclareOption{hide}{\global\@hiddentrue}
\ProcessOptions\relax
\newsavebox{\hidebox}
\newcommand{\showtext}{\@hiddenfalse}
\newcommand{\hidetext}{\@hiddentrue}
\NewEnviron{Hidden}{%
  \par
  \if@hidden
    \begin{lrbox}{\hidebox}
    \begin{minipage}{\linewidth}
      \strut\BODY\strut
    \end{minipage}
    \end{lrbox}
    \vspace{\dimexpr\ht\hidebox+\dp\hidebox-\baselineskip}
  \else
    \BODY
  \fi}
\end{filecontents*}

\usepackage[hide]{MyPackage}

\begin{document}

This should always show up.
\begin{Hidden}
\lipsum[1-2]
\end{Hidden}
This should always show up.

\clearpage
\showtext

This should always show up.
\begin{Hidden}
\lipsum[1-2]
\end{Hidden}
This should always show up.

\end{document}