This is a variant uniform I thought of several years back (September 1, 2007 to be exact) allowing the tunic to be more of a dressier frock over a black, high-collared undershirt. Padding the chest flap could help compensate for a bit of a stomach which afflicts a lot of us in our "middle age." It is partially inspired by the Cage Jacket as well as the maroon coats used in the movies.
Another idea I've been kicking around is a doeskin suede version of the Cage jacket might be a good update for TOS. It would effectively combine the material used in the ST:TMP field jacket and the Charlie X-worn Starfleet uniform jacket with the pattern of the Cage jacket. The Cage jacket is a good pattern, IMHO, that has design elements later used in the TOS duty tunic as well the monster maroon jacket used for many movies and many years.