Is there a simpler Macro for fraction

The OP has indicated a preference for a pdfLaTeX-based solution. However, since I don't know how to provide such a solution, I'm providing a LuaLaTeX-based solution instead. Maybe readers who don't mind using (or actually prefer to use!) LuaLaTeX will find it useful.

The Lua function is set up to capture and process expressions of the form

{...}/{...}

The mandatory elements for a pattern match to occur are (a) two pairs of matching curly braces, (b) the / symbol, and (c) no whitespace to the left and right of /. If these three elements aren't found, no pattern match occurs, and no `\frac{...}{...} expression is produced.

Edit: The function can now handle nested expressions such as {{a}/{b}}/{{c}/{d}}; that'll produce \frac{\frac{a}{b}}{\frac{c}{d}}.

The code shown below also sets up two LaTeX macros: \InlineToFracStart and \InlineToFracStop. The former activates the Lua function, the latter disables it. Having these macros may be useful, as running the Lua function imposes some overhead in terms of scanning and processing the input lines. For instance, if it's known that the document contains expressions of the form {a}/{b} in sections 2 and 3, but not elsewhere, one could run \InlineToFracStart at the start of section 2 and rund \InlineToFracStop at the end of section 3. (Of course, if you don't mind or care about incurring overhead and just want to have the Lua function cover the entire document, simply run \InlineToFracStart just before or after the \begin{document} statement.)

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{luacode}
%% Lua-side code
\begin{luacode}

function inline2frac ( s )
   s = s:gsub ( "(%b{})/(%b{})" , function (x,y)
                    x = inline2frac ( x )
                    y = inline2frac ( y )            
                    return "\\frac{"..x.."}{"..y.."}"  
                end ) 
   return s
end

\end{luacode}
%% TeX-side code: macros to enable and disable the Lua function
\newcommand\InlineToFracStart{\directlua{luatexbase.add_to_callback(
   "process_input_buffer", inline2frac, "inline2frac" )}}
\newcommand\InlineToFracStop{\directlua{luatexbase.remove_from_callback(
   "process_input_buffer", "inline2frac" )}}

\begin{document}
\InlineToFracStart % enable the Lua function

$\displaystyle {a}/{b} \quad {{a}/{c}}/{b} 
  \quad {{a}/{b}}/{{u}/{{v}/{{w}/{{x}/{y}}}}} \quad a/b$
\end{document}

Without LuaTeX, as the OP indicates as preference, you could use the primitive \over command: {a\over b}. You can then use this to do (almost) exactly what you propose:

Edit: @Manuel points out that making it math active is better:

\documentclass{article}
\begin{document}
\mathcode`\/="8000
{\catcode`/=13\global\let/\over}
A/B % <- no effect
\[ {over the line/under the line} \]
\end{document}

Remember to enclose the slash with braces: {}. E.g. {abc/def} works, but abc/def will probably not work as expected, because everything on the right of the / will now in mathmode become a fraction, unless you use the braces {./.}.

Previous answer:

\documentclass{article}
\begin{document}
\catcode`/=13
\def/{\ifmmode\over\else\char`\/\fi}
A/B \[ {over the line/under the line} \]
\end{document}

---

Beware. In both cases, you'd always get a fraction with the slash operator in math mode.