Hello Timothy !

I found a solution to this problem again using the Notation package and some PatternWrappers and it goes like this:

Needs["Notation`"]

ClearNotations[]

Notation[ParsedBoxWrapper[

FractionBox["1",

SuperscriptBox["",

TemplateBox[{

RowBox[{"Simplify", "[",

SuperscriptBox["a_",

RowBox[{

RowBox[{"-", "b_"}], " ", "c_"}]], "]"}]},

"NotationPatternTag"]]]] \[DoubleLongLeftArrow] ParsedBoxWrapper[

SuperscriptBox["a_",

RowBox[{

TemplateBox[{

RowBox[{"b_", "?", "Negative"}]}, "NotationPatternTag"], " ", "c_"}]]]]

Notation[ParsedBoxWrapper[

FractionBox[

SuperscriptBox["a_", "d_"],

TemplateBox[{

RowBox[{"Simplify", "[",

SuperscriptBox["a_",

RowBox[{

RowBox[{"-", "b_"}], " ", "c_"}]], "]"}]},

"NotationPatternTag"]]] \[DoubleLongLeftArrow] ParsedBoxWrapper[

SuperscriptBox["a_",

RowBox[{

RowBox[{

TemplateBox[{

RowBox[{"b_", "?", "Negative"}]}, "NotationPatternTag"], " ", "c_"}],

"+", "d_"}]]]]

With this expressions with negative exponents are always output as fractions. I am still not sure, if this works in every case, but for me it is doing the trick !