would you like to release a Galaxy chocolate bar with mint flavour?
Are you a Mars representative, by any chance?
I don't really think that the particular creaminess of Galaxy chocolate would well complement a mint flavour, be it fondant filling, yukky, ubiquitous ganache or mint-flavoured chocolate (its difficult to decide which would be worse in this case). As drterror666 said, all would likely result in something rather repugnant.
Non-biscuit-based milk chocolate and mint haven't always fared too well. Aero does ok, but Cadbury Mint Chips didn't last too long (though may have been down to too much choice in the range at once) and neither did McVities mint chocolate digestives (yet Orange are still around). Most mint chocolates from all market spectrums involve a non-creamy, non-milky type of chocolate, which one might say is the opposite of the traditional idea and concept of Galaxy-style chocolate. Mingles, Green and Blacks, Lindt, Bendicks, Elizabeth Shaw, Twilight, After Eight, Chocolate Orange etc. etc. Not that Galaxy isn't lovely in own its place, but both fans and phobics will agree that it is extremely rich and 'creamy', both literally and metaphorically. Indeed this very 'creaminess' is one of its USPs.
If you must do a mint variant it might be wiser (and more tastier/mass-appealing with the current market appeal of dark chocolate ever-increasing) to use a dark, less creamy/sickly chocolate than normal, milk Galaxy chocolate, especially if you are going for a whole, thick bar format rather than smaller, bite-size offerings.
Is there perhaps something different you could concoct, if there is a burning need to revitalise or refresh the Galaxy range? Bringing back Counters was a wonderful move on Mars' part, and they aren't particularly complex or complicated. I'm sure this forum could provide one with endless ideas.
Oh and please bring back the Galaxy truffle acorn? I only dimly remember them but the truffle was the loveliest truffle I've ever eaten.