My ideal tunnel setup would have two stations, each located at the country end of the two termini. The key is to extend the city centre northwards and southwards, rather than keeping it locked in between Argyle Street and Sauchiehall Street. The southern entrance to Central would front onto the Clyde waterfront while the northern entrance to Queen Street would be up at Buchanan Bus Station. One of the advantages of stations further from the city centre is that it'll be easier to dig down and then regenerate, rather than needing to replace existing development in and around Buchanan Street. In my mind I hope that the Buchanan Galleries work doesn't go ahead until a tunnel plan is drawn up. That whole area around Queen Street could be transformed by a tunnel project.