Alternatives to footnotes?

You can use end notes. Which basically are footnotes collected at the end of the chapter. It depends on how the footnotes are used.

Does the reader need to look up the information instantly? Then endnotes are not a good alternative. As you already suggested, margin notes can be used, instead.

Try using margin-notes by employing the xcolor, marginfix and the perpage packages:

\usepackage{perpage}
%%%%% Randnoten mit marginfix und perpage
\newcounter{Rnnr}
\MakePerPage{Rnnr}% Zähler beginnt neu auffe nächste Seite, Paket
                  % perpage, es gibt noch zref-perpage
\renewcommand{\theRnnr}{\alph{Rnnr}}
\newcommand{\Anm}[1]{%
  \stepcounter{Rnnr}%Randnotennummer um eins erhöhen
  \textcolor{red}{\textsuperscript{\textit{\theRnnr}}}% Anker im Text
   \marginpar{% Randnotiz
    \RaggedRight% Flattersatz
    \textcolor{red}{\sffamily{\textsuperscript{\theRnnr}}}% Randnotennummer
    \scriptsize% kleine Schrift
    \textcolor{red}{#1}}}%Randnotentext

Instead of \footnote try \Anm. This small hack gives you margin notes "numbered" with small roman letters.

Don't mind my German comments, if you need help, I will translate them for you.