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 !
